Supertex inc. AN-H23 Application Note Avoiding Turn-on Oscillations in the HV91 Family of PWM ICs Introduction Since there is no hysteresis in the undervoltage shutdown in the Supertex HV91 series of converters, there may be situations where insufficient power is provided to the VDD terminal of the chip by the power supply on its first cycle to replace the charge that was removed from the VDD-to-VSS capacitor to drive the MOSFET on its first cycle. As a consequence of the lower VDD caused by driving the MOSFET, the undervoltage shutdown is triggered and the PWM shuts off, until the energy in the capacitor is replenished via the internal depletion-mode starting FET. This condition is more prevalent when the HV91 must drive a large MOSFET, and the VDD capacitor is an electrolytic and located some distance from the IC, or is heavily loaded externally, as can be the case when the VDD supply for the PWM is also used by a load the power supply is powering. If the previous solution is not possible, there is a second solution that is almost as simple, but it requires a bit more calculation. There are a couple of methods of preventing this turn-on oscillation. The simplest is to use a capacitor with very good high frequency performance and that is sized to slightly more than 100 times the effective gate capacitance (not the CISS) of the MOSFET being driven. Mount it very close to the VDD and VSS terminals. To determine the effective gate capacitance of the MOSFET, find the total gate charge on the “Gate Charge vs. Gate Voltage” graph on the MOSFET data sheet and, using the VDD of the HV91 as a gate voltage divide gate charge by gate voltage to determine effective gate capacitance. (It will be a larger value than CISS.) A good stacked film capacitor of 100 times the effective gate capacitance should prevent the oscillations. Figure 2: A Turn-On Delay VDD Stacked film capacitor C ≥ 100 x Qg VDD HV91 VSS VDD Supply C (unchanged) HV91 1N4148 VSS Shutdown A diode may be added for faster discharge. To shutdown circuitry (if used) COM Delay C The shutdown terminal of the HV91 has an internal current source that is normally used only to keep the pin high so that it can be ignored when remote shutdown is not used. This current source will source approximately 50% of the current being drawn from the bias pin by the bias setting resistor (see Figure 3 on the data sheet to determine IC bias current). Placing a delay capacitor on the shutdown terminal will delay the normal turn-on of the HV91 until VDD has risen high enough so that the capacitor on the VDD line can supply the power necessary to drive the MOSFET without VDD falling so low that the undervoltage turns the PWM off again. To determine the size of capacitor required, first calculate how long it will take for VDD to rise to where it will not be turned off by a slight droop. Generally this time (in seconds) will be 500 to 1000 times the value of the capacitor (in farads) connected between the VDD and VSS pins. The value of the delay capacitor then can be calculated from: tDELAY • IBIAS CDELAY = VDD Total lead length should be < 2cm Figure 1: The Easy Way Doc.# DSAN-AN-H23 A040313 Supertex inc. www.supertex.com AN-H23 To discharge the delay capacitor, many schemes can be used. Two of the simplest are to use a diode from shutdown to VDD (though this can be disrupted by very short drops in power) or to use an active discharge. VDD HV91 Supply C (unchanged) If rapid power cycling is expected, an active discharge such as that shown in Figure 3 can be used. This circuit uses a 2N6027 programmable unijunction transistor, which, when VDD drops approximately 0.7V below the voltage on the delay capacitor, quickly discharges the delay capacitor. 47k VSS Shutdown To shutdown circuitry (if used) A Delay C K COM G 2N6027 P.U.T. Figure 3: Active Discharge Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives an adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and specifications are subject to change without notice. For the latest product specifications refer to the Supertex inc. (website: http//www.supertex.com) Supertex inc. ©2013 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited. Doc.# DSAN-AN-H23 A040313 2 1235 Bordeaux Drive, Sunnyvale, CA 94089 Tel: 408-222-8888 www.supertex.com