pulse transformer mosfet drive....?

[s.manikandan]

i m designing a PWM controlled full bridge converter for my application. I have a pwm controller which has 200mA output current to drive mosfet switch. But for my mosfet i need a 1.5A peak current gate current.

I decide to drive using PULSE TRANSFORMER ,but how to achieve this current. is there any way....? without using external driver....

 

[mtwieg]

It's normally necessary to use external drivers for high power supplies. Though you may be fine with a discrete driver (like a push pull stage made of BJTs) rather than a driver IC.

 

[Tunelabguy]

i m designing a PWM controlled full bridge converter for my application. I have a pwm controller which has 200mA output current to drive mosfet switch. But for my mosfet i need a 1.5A peak current gate current.

I decide to drive using PULSE TRANSFORMER ,but how to achieve this current. is there any way....? without using external driver....

I don't know how a pulse transformer is going to help you, other than by providing isolation. Is the power MOSFET sinking to ground or is it high-side switching? If it is sinking to ground then your gate driver does not need isolation. You mention the gate drive requirement of 1.5A. Are you sure it is a requirement, or just a maximum rating for the MOSFET? Normally MOSFET gates are ultra high resistance, needing almost no gate current at all to keep the switch on. However the gate exhibits a very large capacitance, so a feeble gate driver will take a long time to charge that capacitance while a high current gate driver can charge the gate capacitance very quickly. And while the gate is charging, the gate voltage is not allow the switch to fully turn on. Therefore the MOSFET conducts some current and dissipates a lot of heat. It could even burn up. But if MOSFETs spend most of their time either fully on or fully off then they can run quite cool, and may not even need a heat sink. So the considerations for designing the gate drive should take into account the expected switching frequency as well as the switched current. You cannot do it just by looking at the gate maximum current rating. Depending on your application, the gate drive may be simpler than you think.

On the other hand if you have already done this calculation and are sure you really need 1.5 A gate drive then a transformer can only deliver more current if it is supplied with more voltage. If your 200 mA output is available at 80 V then you could use a 8:1 pulse transformer to produce 10 V at 1.5 A.

 

[Tahmid]

By using a pulse transformer, you can't amplify current (assuming you have a turns ratio of 1:1). If you increase current, you'd have to decrease voltage and vice versa.
You should add a driver stage between the PWM controller output and the transformer. You can use a BJT totem-pole stage or you can use a dedicated low-side MOSFET driver chip.

Hope this helps.
Tahmid.

 

[s.manikandan]

I don't know how a pulse transformer is going to help you, other than by providing isolation. Is the power MOSFET sinking to ground or is it high-side switching? If it is sinking to ground then your gate driver does not need isolation. You mention the gate drive requirement of 1.5A. Are you sure it is a requirement, or just a maximum rating for the MOSFET? Normally MOSFET gates are ultra high resistance, needing almost no gate current at all to keep the switch on. However the gate exhibits a very large capacitance, so a feeble gate driver will take a long time to charge that capacitance while a high current gate driver can charge the gate capacitance very quickly. And while the gate is charging, the gate voltage is not allow the switch to fully turn on. Therefore the MOSFET conducts some current and dissipates a lot of heat. It could even burn up. But if MOSFETs spend most of their time either fully on or fully off then they can run quite cool, and may not even need a heat sink. So the considerations for designing the gate drive should take into account the expected switching frequency as well as the switched current. You cannot do it just by looking at the gate maximum current rating. Depending on your application, the gate drive may be simpler than you think.

On the other hand if you have already done this calculation and are sure you really need 1.5 A gate drive then a transformer can only deliver more current if it is supplied with more voltage. If your 200 mA output is available at 80 V then you could use a 8:1 pulse transformer to produce 10 V at 1.5 A.

Thanks to you a good explanation.. As you said its gate capacitance needs such a peak current to minimize the switching on period, actually its a SG3525A (15A,200mA) pwm controller ,i am going to use for full bridge conventional converter.. so four switch's drive in a alternative pair.. so how to drive the four switch using external driver .. either i should go for two half bridge mosfet driver or a single driver.. Most of everybody driving four switches using two drivers .... do you suggest any driver if you find that is good......

 

[FvM]

The answer depends on the intended PWM scheme which hasn't been mentioned in your posts yet.

 

[vovando1985]

i need pulse transformer from 23vdc-315vdc , can you help me . thank you
yes