Electronic Design

Forward Converters Drive Power MOSFETs At Up To 500 kHz

Simply by combining two simple circuits, high-gate-capacitance MOSFET transistors can be driven at up to 500 kHz with galvanic isolation (Fig. 1). The circuit consists of two forward converters—one converter to charge the gate capacitance and the other converter to discharge it. The recommended supply voltage is 12 to 15 Volts.

At the rising edge of the driving signal, transistor M1 receives a short pulse, typically of 200-ns duration with the values of Rf1 and Cf1 shown, and transmits it to the secondary of transformer T1. The reflected current charges the gate-source capacitance of a power MOSFET when transistor Ms1 and diode Ds1 are properly polarized and then closes the current path.

At the falling edge of the driving signal, transistor M2 receives the pulse and transmits it to the secondary of transformer T2. Now, transistor Ms2 and diode Ds5 are properly polarized, opening a path through which the gate-to-source capacitance of the power MOSFET is forced to discharge.

When no signal is transmitted by transformers T1 and T2, the gate-tosource voltage of the power transistor is completely blocked. As a result, it’s neither charged nor discharged and therefore the power MOSFET operation point is maintained.

The third winding of the transformers is used to discharge their magnetizing currents, allowing them to operate at high frequencies without saturation. Transformers T1 and T2 are built using Siemens N67 P11 x7 cores with three interleaved windings (to decrease the leakage inductance) of 10 turns each.

The waveforms shown illustrate the operation of the circuit at 500 kHz when driving an IRFP250 MOSFET device (with a gate capacitance of over 3 nF for VDS = 10 V) (Fig. 2). The upper waveform is the current through the primary of one of the transformers at 100 mA/div. The bottom trace is the gate-to-source voltage of the driven power transistor at 10 V/div (attenuation is provided by an isolated voltage probe).

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