I am driving a 12V, 10W Bulb and 12V, 5W Bulb (parallel connected) on one channel of high side driver solid state relay IC. For dimming effect, Bulbs are driven by PWM (Intial 3msec) then continuously ON. After some use, the IC channel fails to work.

VNQ600AP-E IC spec:
Output current (continuous), for each channel IOUT 15A
MAX ON-state resistance Ron 35mΩ
Current Limitation Ilim 25A
Bulb inrush current is below 25A.

Can anyone please give explanation, why the IC is getting damaged.

  • \$\begingroup\$ IC part number? \$\endgroup\$ – alexan_e Feb 10 '14 at 8:20
  • \$\begingroup\$ link to IC datasheet (st.com/st-web-ui/static/active/en/resource/technical/document/…) @Russell \$\endgroup\$ – Akanksha Feb 10 '14 at 10:42
  • \$\begingroup\$ What is the PWM frequency you are using and what is the minimum duty cycle? \$\endgroup\$ – alexan_e Feb 10 '14 at 10:56
  • \$\begingroup\$ That's a nice IC. I'd guess something like inductive spike was the most likely reason. Try my diode suggestion. Also the series resistor test can do no harm. Report back ... \$\endgroup\$ – Russell McMahon Feb 10 '14 at 17:43
  • \$\begingroup\$ PWM frequency is 2KHz and minimum duty cycle is 5% @alexan_e \$\endgroup\$ – Akanksha Feb 12 '14 at 10:51

Driver IC part number and link to datasheet would be useful.

Bulb link too possibly. Inrush currents can be immense.

You do not say what driver voltage rating is.

  1. Less likely

There MAY be an inductive component.
This is not obvious, but a reverse diode across and near loads would check that.

  1. More likey.

Inrush currents for cold bulbs can be immense. Unless your figure of 25A is from measurement rather than form a specification it could be higher. Having the two bulbs in parallel MAY cause one to come up to temperature slowly if one hogs the current as they heat differentially. This seems unlikely, but ... .

An easy check is to add a series resistor that guarantees maximum driver continuous current rating is not exceeded.
1 Ohm in series is very safe.
0.8 Ohm in series should be safe.
0.5 Ohm in series may well be safe.

See below for why ...

Current limiting resistor:

Load effective on current = Power/Von = (12+5)/12 ~ 1.5A.
Effective on resistance = Von^2/P) = 144/17 ~ 8.5 Ohms.

To limit driver to 15A max at turn on with ~ 0 cold resistance
R = V/I = 12/15 = 0.8 Ohms.
This will reduce bulb operating current to ABOUT V/R = 12 / (8.5 + 0.8) ~ 1.3A and power down to VI = 12 x 1.3 = 15.6 (from the original 17.)

I said "about" as the lower current leads to higher bulb resistance leads to lower current leads to .... .
So maybe a series 0.5 Ohms.
But a series 1 Ohm would be very safe and provide a good starting position.

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  • \$\begingroup\$ For Driver IC Operating voltage range : 5.5V-13V(Typ) -36V \$\endgroup\$ – Akanksha Feb 10 '14 at 6:55

What is the turn-on time of the SSR? It's possible that the PWM is causing problems if the frequency is too high because the power device spends too much time partly "on", violating Safe Operating Area restrictions.

Edit: It appears the device is not an SSR but a protected high-side driver. In that case, I suspect something other than the lamp load may be causing the failures. What value of TVS (hold-off voltage and rating) have you used for \$D_{ld}\$ ?

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  • \$\begingroup\$ link to IC datasheet (st.com/st-web-ui/static/active/en/resource/technical/document/…) \$\endgroup\$ – Akanksha Feb 10 '14 at 10:44
  • \$\begingroup\$ Dld used has breakdown voltage VBR 30V at 1mA, Clamping voltage VCL 48.3V at 103A \$\endgroup\$ – Akanksha Feb 10 '14 at 11:22
  • \$\begingroup\$ Abs max \$V_{CC}\$ is 41V, so it's not quite protected. Still, at this point, I'd be talking with ST's application folks. They may be able to do a failure analysis on your parts. \$\endgroup\$ – Spehro Pefhany Feb 10 '14 at 11:25
  • \$\begingroup\$ Can you elaborate - "it's not quite protected" \$\endgroup\$ – Akanksha Feb 10 '14 at 11:37
  • \$\begingroup\$ By my calculations, 48.3 > 41. Of course your current may be limited to well under ~100A, so perhaps it is effectively protected. \$\endgroup\$ – Spehro Pefhany Feb 10 '14 at 11:41

PWM frequency is 2KHz and minimum duty cycle is 5%

2KHz has a period of 500us, 5% duty is a pulse with a duration of 25us but according to the datasheet the turn on and turn off time (typ) is 40us (with specific load specs but still the drive and delay difference is noticeable). Maybe that is the cause of the problem.

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