2 Sw Fwd/Flyback - high side GD power!

Getting power to the high side of a 2 switch forward ( or flyback ) - can be very problematic at startup.

Switching the bottom fet is no problem - but energy only gets to the high side gate drive power via the bootstrap diode D5, and caps C1,C3 via the transformer primary - which is a high impedance, and very slow, path.

Adding 4 components ( L2, D1, D7, R1 ) in the form of a current limited boost circuit - gets energy to the high side gate drive very much faster - and contributes energy to steady state gate drive power.

You can see that current builds up in L2 via M1 and D7, and that the energy in L2 goes through D1 to the high side caps at M1 turn off.

[ R1 limits the current for max D on the switches ]

This builds up the voltage on the high side caps ( C1, C3 ) until the UVLO on the high side gate driver allows the high side mosfet, M2, to turn on for the same times as M1.

As soon as M1, M2 are switching together, there is another path for L2 current after turn off, through the high side caps to the source of M2 and then backwards through D4 ( i.e. subtracting from D4 forward current ). The standard bootstrap parts D5, R2 contribute to the ramp up of high side Vcc as soon as there is forward current in Tx1 at turn off.

This then allows the nice usual soft start PWM on the switches - with the high side Vcc caps getting to 10V in as little as 200uS.

As soon as the high side starts switching - the Vcc - high side - gets to the zener limit in a little more than a hand full of switching cycles afterwards.

N.B. here we have used 80kHz switching ( 6.25uS half period ) with the pwm sweeping to 5uS on time.

Another great circuit for power electronics that just works, from:

pwrtrnx.com

[ p.s. a lossy ferrite bead on D1, D5, D7 is a great idea for EMC ]

[ p.p.s. a similar system can be used for high side Vcc for synchronous reluctance motor drives and BLDC motor drives ]

Fig. 1 Plot of high side Vcc @ 2uS constant on time, note kick in of high side mosfet @ 500uS, and Vcc at 15V, 100uS after that.