Description:TECHNICAL FIELD
[0001] The present disclosure relates to transformers in induction heating systems, and more particularly, to a pulse-transformer for isolated gate driving of parallel connected SiC Mosfets in induction cap sealing applications.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present disclosure. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed disclosure, or that any publication specifically or implicitly referenced is prior art.
[0003] Induction heating principle is used for sealing of bottles filled with diverse products, e.g., pharmaceutical, petroleum, nutraceuticals, food and beverage items, etc. Using the energy transferred contactless to thin Aluminium (Al) foil the induction sealing ensures the bonding of foil with container’s lip. The induction sealing process may use a continuous-run controller or may be switched ON when desired. A single-switch power controller is used for ON-OFF mode controller, where a parallel resonant tank circuit is energized every switching cycle by large-current short pulse to enable transfer of requisite energy to the foil. To increase the productivity in ON-OFF mode, as required by capless sealing process, both ON and OFF time of the controller are kept small. Essentially, high-speed induction sealing in ON-OFF mode requires higher capacity controller with better thermal management.
[0004] Sealing of plastic or glass containers could be the most popular application domain of IH (Induction Heating) principle. Illustrated in Fig 1, is a representation (100) of a set of sealed glass and plastic containers. As shown in Fig. 1, the transferred energy to a thin Al foil is used for sealing containers having different foil dimension.
[0005] Illustrated in Fig 2, is a representation (200) of a typical single-switch controller used for industrial sealing process. The high-speed sealing process is set on-line where the power converter is continuously kept on. For sealing smaller quantity, an ON-OFF type single switch controller is used. The speed of production could be increased if a power rating of the controller is increased, and/or through an improved thermal design. Often, a capless induction sealer uses a high-speed sealer in an ON-OFF mode. In modern packaging, the role of a capless induction sealer is important. In traditional induction cap sealing process, using paraffin wax, a cardboard placed inside the cap is used to hold a thin foil. The mandatory wax removal process slows down the speed of sealing to a certain extent. It is difficult to perform quality check of each sealed containers whose top is covered by a cap. The process could generate quality issues like overheating/burning on certain area of the foil, under-sealing or less heating, and incomplete wax removal.
[0006] The mechanism of checking the quality of sealing of each container indirectly is not only cumbersome, but also error-prone, extremely costly and sluggish. In order to have better and direct quality check of each sealed container in a simple, and cost-effective way, the emerging trend is to migrate to capless induction sealing where the role of power converter is critical. Ideally, it should be compact and light weight where single switch topology could be ideal. One typical single-switch induction heating controller is shown in Fig. 2. Minimally, its tank-circuit consists of inductor L1 and capacitor Cr. In this automated process the coil L1 is energized only when the coil head holding a bare foil is perfectly engaged to the lip of the container.
[0007] For high-speed sealing both the ON and OFF time of the controller is kept small, almost like a continuous duty use. The converter does not have any power control circuit. The quantum of energy transferred to the foil is controlled by changing the duration of ON time of each sealing cycle. The bulk of power loss in the converter takes place in L1 and Q1. For L1, it is spatially distributed. Whenever energized, the presence of foil underneath significantly reduces the impact of proximity effect on its power loss. The reduction of power loss in Q1 is a concern.
[0008] The power POUT transferred electro-magnetically to a foil is expressed as, , where L1 is inductance of coil head and iL is the coil current of frequency fs. Req represents load resistance. Rfoil of a foil reflected to the tank circuit is expressed as Lfoil is foil inductance and Rfoil is the resistance of the foil, M is mutual inductance between the coil and the foil and .
[0009] For a particular value of Rfoil, the value of Req could be increased by increasing the frequency fs. When the coil holding the foil is energized, current drawn by the foil would reduce the value of inductance from L1 to, say, Leq, like, The loading of coil reduces the value of L1, while effective resonant frequency fs of the tank circuit is increased, because,
[0010] The frequency of ON-OFF operation of an induction sealer is desired to be high. The induction sealer needs to be equipped like a continuous run converter. Loading pattern of a power converter is complex. The value of Req varies widely. Req is zero at no load when there is no foil beneath the coil. Req changes with the diameter of the foil. Depending upon the load conditions, the value of time constant t of current envelope drifts between its maximum and minimum values, like,
[0011] Here, rac is the ac resistance of the coil L1. When compared with loading of other applications (e.g., induction cooking), due to very thin (10-20 µm) Al foil considered as load, the Req is not large; the condition is true. For such load range, the current iL would be sinusoidal, as illustrated in Fig. 5, and remain continuous even by momentary charging of capacitor Cr in each cycle. The expression of voltage across Q1 is, .
[0012] Charging of a tank-circuit should not a incur large power loss in Q1. To eliminate turn-on loss, if Q1 is turned on at zero voltage then the power is transferred to the tank circuit mostly via L1. The peak value of current light-load applications could be large, predominantly decided by rac and the current limiter as illustrated in Fig. 2. An associated large power loss in Q1 could either effect a thermal shut-down of the converter or damage the switch. Moreover, if value of VCE is large at turn off, then there would be significant power loss during turn-off. For uncertain wide range value of as load, the requisite energy Etank is transferred to the tank circuit predominantly through charging of Cr in each cycle of frequency fs. It is possible when the voltage VCE is positive, i.e., near zero voltage switching condition. Etank could be expressed as,
[0013]
[0014] is the voltage across Q1 at turn-on instant. When Q1 is turned off, the tank-circuit is decoupled from the supply. The waveforms of the converter (Fig. 2) at two different preset values of VCE for turn-on are shown in Fig. 3. The converter uses three high-speed low-loss IGBTs (Table 1). The value of fs of unloaded tank circuit is 53 kHz. The profile of current IQ1 in Q1 for charging the Cr could be derived from , where D is duty cycle in a full cycle period .
[0015] Table 1: Features of the single switch converter: three IGBTs vs two SiC Mosfets.
IGBT SiC Mosfet
Device Q1 STGW40H120DF2 ACM020P120Q
Device rating at Tc=100 0C 40A, 1200V 71A, 1200V
Gate threshold voltage, Vgth, V 6.0 1.8
Switching frequency fs, kHz 60 60
No. of devices in parallel 3 2
Peak current per device, A 36 54
Total power loss in Q1, W 112 52

[0016] The selection of a device for Q1 is based on power loss, voltage stress, temperature rise, ease of gate driving, and cost of the device. Cost includes the price of the device, associated heat sink and gate drive circuit (GDC), and area of PCB (Printed Circuit Board) consumed. In each switching cycle the peak value of VCE appears when Q1 is in turned-off condition (Fig. 3). Voltage rating of the device should be 1200V. A turn-on time of Q1 in each cycle is small, ideally, just sufficient to charge Cr to VDC. Peak device current for charging Cr is large. Though there could be a certain voltage across Q1 during turn off, yet the current through it would ideally be small.
[0017] The power loss PL in Q1 consists of conduction loss Pcond and switching loss Psw in:
[0018] Pcond depends on conduction drop, current profile and pulse duration . The switching energy loss Esw consists of turn-on loss Eon, turn-off loss Eoff in Q1 and reverse recovery loss Err in anti-parallel diode. The value of each loss component depends on the value of current and voltage at the switching instant and the junction temperature Tj. For effective transfer of power Pout of desired value, the value of fs is kept large. It is decided by the tank circuit parameters L1 and Cr. The turn-on time DTp of Q1 is kept small. The Tj of a device is expressed as, , where Tamb is ambient temperature, Rth is thermal resistance of the device and heat sink.
[0019] Illustrated in Fig. 4 is a graphical representation (400) of waveforms of the inverter including the current through Q1 for two different sealing applications. The value of peak current was 108 A. Three number of high-speed low-loss IGBTs (STGW40H120DF2) were used for Q1. They were driven by a single non-isolated unipolar gate drive circuit (GDC). The value of L1 was 18µH and that of Cr was 0.5µF. In each cycle, Q1 was turned on for 3µs. The calculated value of PL of the IGBT assembly for Q1 is listed in Table 1. Fig. 4 also discloses that if the IGBT assembly is replaced by two SiC Mosfets, there would be drastic reduction in power loss.
[0020] A SiC Mosfet based converter would be more appropriate for near continuous-duty needed by automated capless induction sealing applications. Though they appear to be superior for high-frequency large-current pulse charging applications, there could be increased complexity to design a GDC for generating noise-free desired asymmetrical gate voltages with narrow pulse width accurately to drive multiple SiC Mosfets in parallel.
[0021] The SiC Mosfet based converter should have features like proper galvanic isolation for effective use of Kelvin source terminals to decouple the power circuit from the sensitive low-voltage control circuit, an ability to feed bi-polar asymmetrical gate voltage devoid of any ringing or oscillations, and negligible propagation delay and rise and fall time. The SiC Mosfet based converter should also be simple, low cost with minimum component count.
[0022] There is, therefore, a requirement to obviate the above-mentioned problems of prior art by providing a pulse-transformer for isolated gate driving of parallel connected SiC Mosfets in induction cap sealing applications that will reduce power loss and improve the thermal features.

OBJECTS OF THE PRESENT DISCLOSURE
[0023] Some of the objects of the present disclosure, which at least one embodiment herein satisfy are as listed herein below.
[0024] A general object of the present disclosure is to obviate the above-mentioned limitations, by providing a pulse-transformer for isolated gate driving of parallel connected SiC Mosfets in induction cap sealing applications.
[0025] The main object of present invention is to provide a pulse-transformer for isolated gate driving of parallel connected SiC Mosfets in induction cap sealing applications for sealing of bottles filled with diverse products, e.g., pharmaceutical, petroleum, nutraceuticals, food and beverage items, etc by applying the induction heating principle.
[0026] Another object of the present invention is to reduce power loss by providing a pulse-transformer for isolated gate driving of parallel connected SiC Mosfets in induction cap sealing applications.
[0027] Another object of the present invention is to improve thermal management by providing a pulse-transformer for isolated gate driving of parallel connected SiC Mosfets in induction cap sealing applications.
[0028] Various objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like features.

SUMMARY
[0029] Aspects of the present disclosure relate to transformers in induction heating systems, and more particularly, to a pulse-transformer for isolated gate driving of parallel connected SiC Mosfets in induction cap sealing applications.
[0030] An aspect of the present disclosure pertains to an induction cap sealing system.
[0031] The induction cap sealing system comprises of at least two SiC Mosfets connected parallelly whose gates to be driven in an isolated manner. The at least two SiC Mosfets are configured to function as a single switch for induction heating power controller whose gates are driven by maintaining the desired bi-polar asymmetrical gate voltage for at least two SiC Mosfets. The induction cap sealing system further comprises a pulse transformer-based gate drive circuit.
[0032] The pulse transformer-based gate drive circuit is provided with three windings of high self-inductance and coupling coefficient configured to enable noise free gate driving of the at least two SiC Mosfets operating in parallel.
[0033] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS
[0034] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure. The diagrams are for illustration only, which thus is not a limitation of the present disclosure.
[0035] In the figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label with a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.
[0036] FIG. 1 illustrates an exemplary diagram (100) of a set of sealed glass and plastic containers, in accordance with embodiments of the present disclosure.
[0037] FIG. 2 illustrates an exemplary circuit diagram (200) of a single-switch converter for induction cap sealing, in accordance with embodiments of the present disclosure.
[0038] FIG. 3 illustrates an exemplary graphical diagram (300) of waveforms of an IGBT based single-switch power controller for induction sealing when Q1 is turned on at, a) 96V and, b) 152V, in accordance with embodiments of the present disclosure.
[0039] FIG. 4 illustrates an exemplary graphical diagram (400) of Waveforms of IGBT based power controller while sealing, a) 55 mm dia. foil, and, b) 95 mm dia. foil. [for current limiter a 0.8 µH inductor was used], in accordance with embodiments of the present disclosure.
[0040] FIG. 5 illustrates an exemplary circuit diagram (500) of a PT based GDC for driving two SiC Mosfets in parallel, in accordance with embodiments of the present disclosure.
[0041] FIG. 6 illustrates an exemplary circuit diagram (600) of even when the coil was not loaded, large ringing in gate voltages during turn-off effected spurious turn-on of SiC Mosfets while using, a) PT1, and, b) PT2, in accordance with embodiments of the present disclosure.
[0042] FIG. 7 illustrates an exemplary diagram (700) of three separate winding wires that would have leakage flux around, and the leakage inductance as well, and, b) 3-conductor litz-wire like single winding that would have ideally zero leakage flux, hence minimum leakage inductance, any conductor could be used as primary or any secondary.
[0043] FIG. 8 illustrates an exemplary diagram (800) of conductors of the three windings that are wound one after the other (see Fig. 7a), and, c) The proposed PT4: 3-conductor litz-wire wound as a single winding (see Fig. 7b), in accordance with embodiments of the present disclosure.
[0044] FIG. 9 illustrates an exemplary diagram (900) of complete experimental set up, in accordance with embodiments of the present disclosure.
[0045] FIG. 10 illustrates an exemplary diagram (1000) of Control, gate drive and power circuit in one printed circuit board, in accordance with embodiments of the present disclosure.
[0046] FIG. 11 illustrates an exemplary diagram (1100) of primary side waveforms of the proposed PT4 for an input pulse having different duty cycle to prove its wider prospects in induction cap sealing.
[0047] FIG. 12 illustrates an exemplary diagram (1200) of Waveforms of two SiC Mosfets based power converter for induction sealing using the proposed PT4 when the coil was loaded, a) for sealing 55mm foil, and, b) for sealing 95 mm dia. foil, in accordance with embodiments of the present disclosure.
[0048] FIG. 13 illustrates an exemplary diagram (1300) of Sealed containers, a) foil diameter: 55mm, and b) foil diameter: 95 mm, in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION
[0049] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0050] If the specification states a component or feature “may”, “can”, “could”, or “might” be included or have a characteristic, that particular component or feature is not required to be included or have the characteristic. Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. This disclosure may however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the disclosure to those of ordinary skill in the art. Moreover, all statements herein reciting embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future (i.e., any elements developed that perform the same function, regardless of structure).
[0051] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing. The present disclosure relates to transformers in induction heating systems, and more particularly, to a transformer for isolated gate driving of parallel connected SiC Mosfets in induction cap sealing applications.
[0052] While embodiments of the present invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions, and equivalents will be apparent to those skilled in the art, without departing from the spirit and scope of the invention, as described in the claim.
[0053] In the foregoing description, numerous details are set forth. It will be apparent, however, to one of ordinary skill in the art having the benefit of this disclosure, that the present disclosure can be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, to avoid obscuring the present invention.
[0054] While the foregoing describes various embodiments of the disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof. The disclosure is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the disclosure when combined with information and knowledge available to the person having ordinary skill in the art.
[0055] Embodiments of the present disclosure provide an induction cap sealing system. The pulse transformer-based gate drive circuit is provided with three windings of high self-inductance and coupling coefficient configured to enable noise free gate driving of the at least two SiC Mosfets operating in parallel. The single-switch induction heating power controller is an almost continuous duty converter with a low OFF time. The three windings of the pulse transformer-based gate drive circuit possess negligible value of leakage inductance to prevent spurious turn-on of the at least two SiC Mosfets caused by oscillations or ringing in gate voltage. The three windings of the pulse transformer-based gate drive circuit comprise conductors bunched and twisted into a litz-wire. The three windings of the pulse transformer-based gate drive circuit are configured for effective use of Kelvin source terminals of the at least two SiC mosfets. The three windings of the pulse transformer-based gate drive circuit are positioned on a transformer core composed of toroidal shaped ferrites of high permeability. The pulse transformer-based gate drive circuit generates a noise-free asymmetrical gate voltage with a narrow pulse width..
[0056] It is clear from two experimental results of Fig. 4 that the IGBTs were turned-off at zero current but at a certain voltage across Q1. Though multiple (three) IGBTs were driven by a common unipolar GDC, gate signals were devoid of any noise or oscillations. On several fronts, a gate requirement of Si devices is different from that of SiC Mosfets. The gate requirement has been standardized for the Si devices. The Si devices are turned-on at a relatively low voltage (=15V). Depending upon power circuit configuration, the Si devices could be turned-off by applying a wide range of negative bias, it could be zero voltage as well as shown in Fig. 4.
[0057] The ratio of limiting value of gate voltage for turn-on (30V) and turn-off (-30V) is 1. There exists a large operating gap between nominal operating and limiting values. Such flexibility makes the Si devices naturally compatible to the PT-based GDC for wide range duty cycle. Generally, just a change in gate resistor is needed to accommodate a suitable alternate.
[0058] On the other hand, gate voltage requirement of the SiC Mosfets are device specific (e.g., refer to data sheets of ACM020P120Q, IMZA120R020M1H, etc.), influenced by several parameters such as gate threshold voltage, transconductance, device capacitances, , etc. The SiC Mosfets are turned-on at high gate voltage (=18V) and turned-off by mandatorily applying a certain negative bias (= -5V). The ratio of limiting value of gate voltage for turn-on (+25V) and turn-off (-10V) is large (>1). Compared to the Si devices, the respective operating boundary is constrained. The GDC should be able to generate the desired asymmetrical gate voltage accurately. As it operates on flux balancing principle in the core, primary-driven completely passive PT-based GDC apparently appears to be less suitable. The complexity further increases when, to avail of Kelvin source terminals, several parallel devices are driven in isolated manner. For proper current sharing by multiple devices, multi-winding PT must ensure equal dynamic gate voltage in each gate. Lastly, due to the small value of gate threshold voltage, the gate voltage should be devoid of any oscillations to avoid spurious turn-on of SiC Mosfets.
[0059] The pulse-transformer (502) based GDC is shown in detail in Fig. 5. A DC blocking capacitor Cb is added to avoid any magnetic saturation in the PT (502) with two secondary windings, its turns-ratio is 1:1:1. Ideally, the primary voltage would be reflected at the secondary terminals. When the device is desired to be turned-on for duration , at supply voltage Vcc, the primary voltage VTR+ of PT (502) could be expressed as, . During turn-off interval , primary voltage VTR- is,
[0060] For majority of the SiC Mosfets, permissible range of turn-on voltage is , and that of turn-off voltage is . When the value of fs changes with load (4) or application, value of D would also shift. Therefore, both VTR+ and VTR- would drift with fs. For induction sealing the converter operates in a frequency range . Considering the on-time DTp is kept at constant at 3 µs, Table 2 shows the operating gate voltage VTR+ and VTR. Though differently, the operating margin of VTR+ and VTR- changes with fs. When fs is small, not only the operating margin for VTR+ is constrained, small value of VTR- could make the gate susceptible to spurious turn-on. Therefore, the GDC needs to be accurate for proper functioning of the converter over a large band of fs.
[0061] Table 2: Asymmetrical gate voltage over the frequency band of induction sealing converter
Frequency fs, kHz 40.0 50.0 60.0 70.0 80.0
Vcc, V 12.5 12.5 12.5 12.5 12.5
Total period Tp, µs 25.0 20.0 16.7 14.29 12.5
Duty cycle, % 12.0 15.0 18.0 21.0 24.0
VTR+, V 22.0 21.25 20.5 19.75 19.0
VTR-, V -3.0 -3.75 -4.5 -5.25 -6.0

[0062] To generate the desired asymmetrical gate voltage accurately over wide frequency range, the multi-secondary PT (502) needs its each winding to possess a large self-inductance, say, Ls and small value of leakage inductance llk. The coupling coefficient of each winding should be large.
[0063] The single-switch power converter (parameters are listed in Table 3) was designed and tested using two commercially available high-frequency PTs from reputed manufacturers. Both are popular for driving Si IGBT or Mosfet. The frequency range of induction sealer falls within the operating frequency band of both the PTs, their parametric details are listed in Table 3. Apparently, the value of k between any two windings of PT1 and that in PT2 was large .
[0064] Table 3: Parameters of the converter for conducting experiments
Inductance L1, µH 18.0
Tank capacitor Cr, µF 0.5
Device Q2, Q3 ACM020P120Q
Gate resistor Rg1, Rg2, ? 5.0
Turn-on duration DTp, µs 3.0
Operating range of fs, kHz 40-80

[0065] Table 4: Parameters of two commercial pulse transformers
PT I PT 2
Model SIRIO T117323 VAC 4097X058
V.µs rating 140 260
Turns ratio 1:1:1 1:1:1
Isolation voltage, kV 4.0 3.1
Lp, Ls1 and Ls2, mH 0.314, 0.311, 0.303 5.8, 5.79, 5.79
lpl, ls1, ls2, nH 335, 360, 380 400, 410, 418
Coupling coeff., k 0.9993 0.9999
Inter winding cap., Pf ˜38 ˜40

[0066] To understand the role of different parameters of magnetically powered GDC to drive the two SiC Mosfets in parallel, a power converter was designed to accommodate both the PTs as shown in Fig. 6. The graph (600) in Fig. 6 represents the waveforms of two gate voltages, and also the current and voltage in Q1 assembly (i.e., Q2+Q3 in parallel, see Fig. 5) while driving a tank circuit for induction sealing (Fig. 2). It is clear that during the turn-off of Q1, there was spurious turn-on that resulted current flow through the devices. Though, the values of both Ls and k were more for PT2, its performance was worse even at lighter load condition. Extra power loss associated with spurious turn-on could harm the device in either case. The reliability of the converter would be less, there could be large associated electro-magnetic noise.
[0067] It is clear from Fig. 6 that despite having a large value of self-inductance in each winding along with excellent coupling coefficient (as shown in Table 3), both PT1 and PT2 were found to be not suitable for driving the SiC Mosfets in parallel even for a single-switch low-noise resonant converter. Due to small gate threshold voltage, at turn off, oscillations in gate voltage caused by parasitic inductance (llk of PT (502) plus the stray inductance of GDC) while discharging the parasitic capacitances Cgs (gate to source) and Cgd (gate to drain) can take a SiC Mosfet into the active region, resulting in spurious turn-on. When PT1 or PT2 was used, in either case during turn-off, there was spurious turn-on of Q2 and Q3 (as shown in Fig. 5) and large ringing was noticed in both the gate voltages, and also in voltage and current of single switch. It is complex to design a PT-based GDC to feed the desired narrow band asymmetrical gate voltage accurately and robustly to turn-on and turn-off the SiC Mosfets an in isolated manner. The PT (502) needs to be close to an ideal one where llk plays critical role for suppressing any noise at the respective gate. The layout of the GDC also plays important role, its stray inductance should be small. The design of the PT (502) should have large value of Ls with negligible llk in each winding.
[0068] It is clear from Fig. 6 that despite having large value of self-inductance in each winding along with excellent coupling coefficient (see Table 3), both the PT1 and the PT2 were found to be not suitable for driving the SiC Mosfets in parallel even for single-switch low-noise resonant converter. Due to small gate threshold voltage, at turn off, oscillations in gate voltage caused by parasitic inductance (llk of PT (502) plus the stray inductance of GDC) while discharging the parasitic capacitances Cgs (gate to source) and Cgd (gate to drain) can take a SiC Mosfet into the active region, resulting in spurious turn-on. When the PT1 or the PT2 was used, in either case during turn-off, there was spurious turn-on of Q1 assembly and large ringing was noticed in both the gate voltages, and also in voltage and current of Q1 assembly. It is complex to design a PT-based GDC to feed the desired narrow band asymmetrical gate voltage accurately and robustly to turn-on and turn-off the SiC Mosfets an in isolated manner. The PT (502) needs to be close to an ideal one where llk plays critical role for suppressing any noise at the respective gate. The layout of the GDC also plays important role, its stray inductance should be small. The design of a PT (502) should have large value of Ls with negligible llk in each winding.
[0069] It was practically analyzed that to ensure current sharing among parallel SiC Mosfets, the PT (502) should have a large value of self-inductance Ls in each winding along with excellent value of k. And for handling gate voltage robustly during transient conditions, the value of llk should be negligible, depending on winding layout, core material etc. The value of llk depends on the leakage flux available in the core window. Understanding the factors that influence the value of llk is important. Its value between any two windings can be determined by using the expression:
[0070] N is the number of turns of the winding, lmt is the mean length per turn, m is the level of interleaving, is the sum of the heights of all windings, is the sum of the widths of the spacing gaps between winding layers, is the bobbin winding breath. For a 1:1:1 ratio transformer, the number of turns N in each winding is, . Considering the value of , the self-inductance of each winding, like llk, also varies with N2, like, , where AL (nH/turn) is the inductance factor of the chosen core.
[0071] Though llk is significantly reduced by reducing the leakage flux through interleaving, a large value of m increases the complexity of windings and increases the values of , and lmt, it gets more complicated for a 3-winding PT. In this research the objective is to design a simple, compact, and low-cost PT-based-GDC that can effectively drive two parallel SiC Mosfets Q2 and Q3 (Fig. 5) through introduction of requisite voltage isolation for proper use of the respective Kelvin source terminals. It does not need large isolation voltage Viso among windings. If the magnetizing current of PT (502) is neglected i.e., when self-inductance Lp of primary winding is large, then the condition is true.
[0072] In the proposed PT (502), instead of interleaving (Fig. 7a), three minimally insulated or single-enameled conductors consisting of one primary (702) and two secondary windings (704, 706) are bunched and twisted to make like a 3-conductor litz-wire (708). The summation of current in the three-conductor litz-wire (708) bunch would always be zero as it ideally does not create any flux around. When the three-conductor litz-wire (708) bunch is wound on the core like a single winding then the value of leakage flux of any winding is inherently cancelled out; the value of llk of each winding would ideally be negligible.
[0073] The choice of core also plays a vital role for design of the PT (502) for the SiC Mosfets. For accurate control of asymmetrical voltage, the values of Lp, Ls1 and Ls2 and k should be large. On the other hand, to reduce the value of Llk, the value of N should be small. Increase in core area Ac and/or the choice of low-loss core material having high value of saturation flux density and relative permeability would help reduce the value of N. To increase the value of k the core should have large AL (nH/turn) where un-gapped core with large value of relative permeability is preferred. Though, nanocrystalline cores would be ideal, but due to the poor availability of miniature size nanocrystalline cores, toroidal shaped ferrites with large value of effective permeability are used here.
[0074] For comparative study, two PTs were designed and manually wound using a coated toroidal core (T1807, material grade: CF197) with core area Ac of 22.2 mm2 and AL value of 4500 nH/turn. The newly constructed PTs are shown in Fig. 8. Various parameters of the PT (502) are listed in Table 4. It is clear that compared to PT3, the value of llk is minimum in the proposed PT4. The value of llk in PT3 is comparable with PT1 and PT2 (as shown in Table 3). The parametric values of PT4 are excellent with virtually having no deviation in winding parameters; primary and secondary windings could interchangeably be used.
[0075] Table 5: Parameters of the PT with different winding layout
PT3 PT4 (proposed)
V.µs rating 100 100
Turns ratio 16:16:16 16:16:16
Winding layout (dia. of wire of each winding: 0.3mm) Wound like a transformer with no insulation layer placed in between Conductors of 3 windings were bunched like a litz wire, then wound like a one winding
Viso, kV >0.3 >0.3
Lp, Ls1, Ls2, mH 1.07, 1.067, 1.067 1.15, 1.15, 1.15
lpl, ls1, ls2, µH 0.32, 0.32, 0.34 0.17, 0.17, 0.17
Capacitance, pF ˜30 ˜30

[0076] After analyzing the results of using PT1 and PT2 from Fig. 6, it was understood that with similar or worse values of llk the performance of the converter using PT3 (see Table 5) would not be any better. Therefore, requisite experimentation was carried out using the proposed PT4. The experimental set up of single-switch resonant power converter is shown in Fig. 9. The major parameters of the power converter are listed in Table 5. Originally, the design and layout of the converter PCB could accommodate PT1 and PT2. Therefore, as shown in Fig. 10, due to different geometrical shape and winding termination, PT4 was kept hanging. The extra inductance value caused by overhang wires from each winding of PT4 were included in llk of Table 4.
[0077] Fig. 11 shows two sets of waveforms of various signals (as detailed in Table 2) of the primary side of PT4 based GDC (Fig. 5) at two extreme ends of operating frequency range of the converter. Each signal closely matched with the calculated results listed in Table 2. The PT-based power converter was validated in traditional cap sealing applications.
[0078] Fig. 12a shows a set of waveforms while performing traditional induction cap sealing using 55 mm diameter foil. The corresponding sealed container is shown in Fig. 13a. Fig. 12b shows waveforms while sealing using 95 mm foil. The corresponding sealed container is shown in Fig. 13b. It is clear from the waveforms of Fig. 12, belonging to two applications, that the PT (502) wound in a single layer using 3-conductor litz-wire type bunching could virtually eliminate the ringing of gate voltage of Q2 and Q3 to avoid their spurious turn on. The results would further be improved when the stray inductances, particularly of Q3 (Fig. 10), are reduced through improved PCB layout.
[0079] Air circulation inside the converter was ensured by a 3-inch fan. For thermal load testing, a metal pan filled with water was considered as load. The set load (equivalent to worst case sealing load) was emulated by adjusting the distance between the coil and the pan. The ON-OFF duty cycle (5.0 sec ON, 0.5 sec OFF) was 91%. After two-hour thermal load cycling, the steady-state temperature recorded in heat sink was 56 0C. For IGBT based system (see Table 1), though the heat was 33% larger, the steady state temperature was high at 80 0C and ambient temperature was 35 0C.

ADVANTAGES OF THE INVENTION
[0080] The present disclosure provides a novel pulse-transformer for isolated gate driving of parallel connected SiC Mosfets in induction cap sealing applications.
[0081] The present disclosure provides a pulse-transformer for isolated and noise-free gate driving of parallel connected SiC Mosfets in induction cap sealing applications for sealing of bottles filled with diverse products, e.g., pharmaceutical, petroleum, nutraceuticals, food and beverage items, etc by applying the induction heating principle.
[0082] The present disclosure provides an induction cap sealing system that reduces power loss by using a pulse-transformer for isolated gate driving of parallel connected SiC Mosfets in induction cap sealing applications.
[0083] The present disclosure provides a new pulse transformer simple gate drive circuit for effectively driving several SiC Mosfets in an isolated manner to avail the benefits of the power devices to achieve improved thermal management for near continuous duty induction cap sealing applications.
[0084] Using a single uni-polar power supply the present disclosure provides a new simple low-cost gate drive solutions suitable for supplying bi-polar asymmetrical gate voltage for noise-free controlled switching of several mosfets in parallel in an isolated manner.
, Claims:1. An induction cap sealing system, characterized in that, the induction sealing system comprising:
at least two SiC Mosfets connected parallelly and configured to function as a single switch induction heating power controller to be driven by a gate drive circuit (GDC) to feed a desired asymmetrical gate voltage accurately in an isolated manner; and
a pulse transformer-based gate drive circuit,
wherein the pulse transformer-based gate drive circuit is provided with three windings of high self-inductance and coupling coefficient configured to enable noise free gate driving of the at least two SiC Mosfets operating in parallel.
2. The induction cap sealing system as claimed in claim 1, wherein the at least two SiC Mosfets possess low switching power loss and low conduction power loss.
3. The induction cap sealing system as claimed in claim 1, wherein the at least two SiC Mosfets have a gate voltage requirement based on parameters comprising gate threshold voltage, transconductance, device capacitances, and .
4. The induction cap sealing system as claimed in claim 1, wherein the single-switch induction heating power controller is an almost continuous duty converter with a low OFF time.
5. The induction cap sealing system as claimed in claim 1, wherein the three windings of the pulse transformer-based gate drive circuit possess negligible value of leakage inductance to prevent spurious turn-on of the at least two SiC Mosfets caused by parameter driven oscillations or ringing in gate voltage.
6. The induction cap sealing system as claimed in claim 1, wherein the conductor of each of the three windings of the pulse transformer for the gate drive circuit is bunched and twisted to make like a single litz-wire.
7. The induction cap sealing system as claimed in claim 1, wherein the three windings of the pulse transformer-based gate drive circuit are configured for effective use of Kelvin source terminals of the at least two SiC mosfets.
8. The induction cap sealing system as claimed in claim 1, wherein the three windings of the pulse transformer-based gate drive circuit are positioned on a transformer core composed of toroidal shaped ferrites of high permeability.
9. The induction sealing system as claimed in claim 1, wherein the pulse transformer-based gate drive circuit generates a noise-free asymmetrical gate voltage with a narrow pulse width.