Isolation MOSFET-driver IC gets improved power efficiency at lighter loads
An isolation driver with feedback boosts 5V to approximately 13V, reducing power consumption.
Marián Štofka, Slovak University of Technology, Bratislava, Slovakia; Edited by Martin Rowe and Fran Granville -- EDN, January 8, 2009
Many modern power MOSFETs reach low values of on-resistance at 5V even when the gate-to-source voltage is 5V. For heavy-duty power MOSFETS and, especially, IGBTs (insulated-gate bipolar transistors), however, engineers prefer gate-to-source voltages of 12 to 15V because the on-resistance of those power switches further decreases at higher gate-to-source voltages. The 17A-rated IRFR024 power MOSFET from International Rectifier, for example, has an on-resistance of 0.075Ω (Reference 1). When the gate-to-source voltage is 12V, the device’s on-resistance drops to 41% of its value compared to a case of a gate-to-source voltage of 5V. At a switching current of 10A, the device dissipates 6W less when the gate-to-source voltage is 12V.
IC1, an Analog Devices ADuM5230 IC isolation driver, can boost a 5V input to a level that’s high enough to drive a MOSFET’s on-resistance to a low value, minimizing power dissipation (Figure 1). At low switching frequencies, however, the IC’s high-side, internal 18V clamping dissipates the energy that the IC draws from the low-side 5V supply (Reference 2).
The ADuM5230’s output is, however, unregulated. Fortunately, this IC has an adjustment pin that you can use to control the duty cycle of the device’s internal PWM (pulse-width modulator) to reduce the duty cycle from a value of 1 to approximately 0.1. The default duty cycle has the value of 0.55 when the adjust pin is open. The lowest value of duty cycle occurs when connecting the adjust pin to the 5V supply. IC2, an ASSR-1219 advanced photo-MOSFET device from Avago Technologies, controls the voltage at the adjust pin. The photo-MOSFET has 0V saturation voltage between its output terminals. As a classical optocoupler has a bipolar phototransistor, using it as IC2 would be less suitable in this case. A bipolar phototransistor has a saturation voltage of 0.4V, and, further, the CTR (current-transfer ratio) of a common optocoupler would decrease significantly when operating close to output saturation. Pulling the voltage at the adjustment pin to the external voltage-supply level comes into account when the high-side output of IC1 has light or negligible loading.
At some point, VISO, the high-side output voltage of IC1, will exceed the value of approximately VZ(IF)+VFLED~13.5V, where VZ(IF) is the voltage of zener diode D1 at IF, the forward current of D2, and VFLED is the minimum forward voltage at D2, the LED of IC2. IC1 exceeds this value, current starts to flow through the D2, and the MOSFET at the output of IC2 becomes conductive. The manufacturer of IC2 designed it for on/off operation and recommends a forward current of at least 0.5 mA (Reference 3).
At signal-level loading of the MOSFET at the output of IC2, a few tens of microamperes of forward current through the LED cause the photo MOSFET’s on-resistance to change from an almost-infinite value to a value of kilohms. The voltage level at the adjust pin then increases, and the duty factor of both the PWM in IC1 decreases. This action establishes an isolated negative-voltage feedback. Thus, the temperatures of both the MOSFET and the LED in IC2 have little effect on the properties of the circuit. At lighter loads, the current drain of the 5V supply is much lower than that of IC1 with its adjust pin open.
Under test, the default supply current of the unloaded IC1 was approximately 94.6 mA. This value decreases to 31.7 mA with the feedback in the circuit. At heavy loading, the high-side output current of IC1 rises to approximately 20 mA, and the duty factor rises automatically to a proper value that’s higher than at the default supply current. Thus, the output voltage is roughly 13.5V within the range of approximately 3.7 to 22.6 mA. The power efficiency of the circuit is 20% or greater. At an output current of 4.5 mA, the power efficiency is 20.5%, and the power efficiency for IC1 is approximately 15%. At a current of 3.7 mA, the circuit reaches 20% efficiency, a value that’s considerably higher than the 13% in IC1 with its adjust pin open.
References
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“IRFR/U024 HEXFET Power MOSFET,” International Rectifier.
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“ADuM5230, Isolated Half-Bridge Driver with Integrated High-Side Supply,” Analog Devices Inc, 2008.
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“ASSR-1218, ASSR-1219 and ASSR-1228, Form A, Solid State Relay (Photo MOSFET) (60V/0.2A/10Ω),” Avago Technologies, July 18, 2007.
