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driver for two-transistor-forward-converter

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Jun 22, 2008
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how to bootstrap using transistors


I am seeking a “Bootstrap” High and Low side gate driver IC for driving the transistors of a (240V mains input) two-transistor-forward-converter.

I have searched many fab houses www sites and find that they have no off-the-shelf offerings for this, -they only do them for half or full-bridge topologies.

I appreciate that I could augment their offerings by adding additional circuitry, -but the amount of additional circuitry is such that it wouldn’t be worth doing,
-and it would be better to just use a pulse transformer high side drive instead.

The IC that I seek (if it exists) would simply receive the single gate drive signal from my PWM controller, and drive the FETs ON/OFF in synchrony, it would also need to provide a third gate drive signal for a Lo side “bootstrap high side capacitor ‘refresh’ transistor”.
-this transistor would simply come ON some 300ns after the two “main” FETs have gone off, -and then go off a settable delay time after it came ON.

I cannot simply use a single bootstrap high side driver…….because…..
The problem with using a single high side driver to just drive the upper transistor, is that during light load times, the low side “demagnetisation” diode would not conduct for long enough to allow the high side bootstrap capacitor to sufficiently re-charge.

Does any reader know of a suitable “bootstrap Hi & Lo side driver IC for a two-transistor-forward-converter?

Its such a common topology that I am amazed none of the fab houses I’ve googled have anything.

forward converter


u can use an transformer Radio 1:1:1..Prim u decouple with an capacitor..need for the dutycycle...u not need an Bias voltage Bootstrap etc..with the tranformer..
But u need a wire with 600V (Tefzel)


well this is more a driver and PWM in one SG3524. this is great for low freq the SG3525 is for high freq like in a car amp or other SMPS.
i have seen just driver chips as well when i find what they are i will let you know.

I assume, that you have reasons to use high side switches in your design. As a first point, you can get single channel high side drivers, some of them including recharge circuits as FAN7085. They may be possibly unsuited for your application, if the existing load won't allow operation of the recharge circuit. In this case, an auxilary 15 V supply on top of the bus voltage would be the obvious solution.

Generally, an important advantage of the integrated high side drivers is their low-voltage lockout for the bootstrap voltage. So if the bootstrap operation fails for any reason, you don't risk to destroy your power switches.


do u have any schematics of ur design? which power range u need.. Input Voltage Out Put voltage and Power?

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