### AN-H23

```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
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
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.