Isolated MOSFET driver has wide duty-cycle range

-April 29, 2004


The main application for the circuit in Figure 1 is for driving power MOSFETs with signals ranging in frequency from 1 Hz to 300 kHz and with duty cycles from 0 to 100%. You achieve this goal by using a coreless pc-board transformer. The switching frequency in most power-electronics circuits ranges from a few hertz to a few hundred kilohertz. To design a coreless transformer-isolated gate drive that can switch in the range of frequencies lower than 300 kHz, you implement the modulation of a high-frequency carrier by a low-frequency control signal. The energy transfer from the primary side occurs through the use of a high-frequency carrier signal of 3 MHz. The control-gate signal couples to the secondary output by the modulation process. The binary counter, IC3, divides the 24-MHz signal from clock-oscillator IC2 by eight to obtain 3 MHz. The true/complementary buffer, IC6, yields two complementary 3-MHz signals with low delay between them. The NAND gates, IC5, implement the modulation process.

The design uses the value of C3 to obtain maximum impedance at the working frequency. A voltage doubler (D1, D2, C4) furnishes the gate-drive voltage. This design uses a 555, IC7, as a Schmitt trigger because of its low power consumption. D3 prevents the energy stored in C6 from discharging into R1. As you can see in Figure 2, when the control voltage is high, a 3-MHz ac signal appears across the transformer primary, thus charging capacitor C5 and energy-storage capacitor C6. The input to IC7 goes high, thus turning on the MOSFET. When the control voltage goes low, the voltage across the transformer primary drops to zero, and the input to IC7 goes low, thus turning off the MOSFET. Figure 2 and  Figure 3 show the control voltage, the voltage across the transformer secondary, and the gate voltage of the MOSFET.

The dimensions of the transformer and the carrier frequency yield a good relationship between the secondary and the primary voltages and minimize the input power of the gate drive. The transformer has a circular spiral primary winding on the bottom of the pc board. The primary winding has 20 turns of 0.3-mm-wide conductor. The circular spiral secondary winding is on top of the pc board. It has 15 turns and a 0.4-mm-wide conductor. For both windings, the conductor thickness is 35 microns, and the outermost radius is 25 mm. The pc board is 1.54 mm thick. Figure 4 shows a frequency plot of the input impedance of the transformer with the secondary winding terminated by C3. The network analyzer shows that the maximum impedance occurs at approximately 3 MHz. Figure 5 is a photograph of a working prototype.

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