Tom Gross
EDN
It can be difficult to generate a high-voltage supply from a medium-voltage input, especially if you need a surface-mount design. It is difficult to find surface-mount components with the necessary specs, especially the transformer and power switches. High-voltage surface-mount capacitors can also be hard to locate. The circuit in Figure 1 generates 100 V from 25 V. The circuit is a typical flyback regulator that uses a couple of well-established circuit techniques to handle the high voltage. The first technique is to insert an n-channel MOSFET (Si4480) in series with s internal power transistor. The cascoded FET stands off the large switch voltage that arises when the switch turns off. The large switch voltage represents the input voltage summed with the reflected output voltage of the transformer's primary. Using the cascode FET not only increases the effective switch-voltage capability, but also eliminates the need for a snubber circuit across the primary of the transformer.
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| Figure 1. | This bootstrapped high-voltage regulator uses all surface-mount components. | |
Of course, the FET actively turns on when the source goes low. But simply using the input voltage to supply the gate voltage doesn't work, because the maximum input voltage (25 V) exceeds the maximum ±20 V limit of the MOSFET's gate-to-source voltage. Therefore, the circuit uses an emitter follower to supply a constant voltage to the gate of the MOSFET. A 1-kW resistor delivers bias current to an 8.2 V zener diode at the base of the npn transistor and provides base current to the npn. This arrangement sets the npn's emitter and the FET gate connected to it at 7.5 V – more than enough voltage for a logic-level FET. The input to IC1 also connects to the emitter through an RC filter. A convenient feature of this circuit is that the IC switch voltage feeds back through the diode to the input (bootstrapping). This connection allows for a small boost in efficiency, negating the loss of efficiency from using a cascoding FET and the emitter-follower circuit. Figure 2 shows the efficiency of the regulator with and without the bootstrapped diode at different load currents.
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| Figure 2. | Bootstrapping the IC switch voltage to the input in the circuit of Figure 1 improves efficiency by about 3%. |
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