TL494 based SMPS help required

[Martin Whybrow]

I'm fault finding a PSU (an argon ion laser PSU) based around a TL494 SMPS
controller. The SMPS is arranged as a single ended series regulator with
both drive outputs of the 494 in parallel driving the switching FETs. The
dead time control pin (pin 4) is at 0V, but the drive output at pins 9 and
10 is showing 100% duty cycle, the data sheet states it should have a 5%
dead time. The oscillator is running, I get a 100KHz sawtooth at pin 5 (Ct)
ramping from 0.4V to 2.6V and the Vref (pin 14) is correct at 4.99V.
Is the TL494 dead? Any other ideas.
TIA,
Martin.

 

[Sam Goldwasser]

I'm fault finding a PSU (an argon ion laser PSU) based around a TL494 SMPS
controller. The SMPS is arranged as a single ended series regulator with
both drive outputs of the 494 in parallel driving the switching FETs. The
dead time control pin (pin 4) is at 0V, but the drive output at pins 9 and
10 is showing 100% duty cycle, the data sheet states it should have a 5%
dead time. The oscillator is running, I get a 100KHz sawtooth at pin 5 (Ct)
ramping from 0.4V to 2.6V and the Vref (pin 14) is correct at 4.99V.
Is the TL494 dead? Any other ideas.

I've seen them die. Given that it's a 50 cent part (if you don't have
a PC power supply to rip one out of!), swapping is probably easiest way
to be sure.

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[techforce]

Yea they fail. Usually Temp sensitive. Try heat and cold on the IC and I bet
you'll see it fire back up.

 

[Martin Whybrow]

I've seen them die. Given that it's a 50 cent part (if you don't have
a PC power supply to rip one out of!), swapping is probably easiest way
to be sure.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.

Sam
I've replaced the 494 (with one from an old PC power supply), still get 100%
duty cycle on the outputs.
I posted more measurements over on alt.lasers yesterday BTW.
Martin

 

[Franc Zabkar]

Sam
I've replaced the 494 (with one from an old PC power supply), still get 100%
duty cycle on the outputs.
I posted more measurements over on alt.lasers yesterday BTW.
Martin

What are the voltages on the error inputs, ie pins 1,2,15, and 16?


- Franc Zabkar

 

[Martin Whybrow]

What are the voltages on the error inputs, ie pins 1,2,15, and 16?


- Franc Zabkar

Franc
Pin 1 at 0V (connected to pin 7 Gnd), pin 2 at 5V (connected to pin 14 Vref)
so EA1 is not used. Pin 16 pegged at + supply rail, pin 15 at 4.8V
(connected to pin 3, comp output). I have now deduced that the FETs are
switched hard on because there is an inverter (DS0026) driving them from the
emitters of the output stage of the 494, hence 100% duty cycle. It appears
that this PSU is designed to work 'upside down', i.e. a + feedback input
turns the FETs hard on. The feedback signal is derived from 2 sources, a
current monitor measuring load current (0 because the laser tube doesn't
ignite) and a light monitor signal and possibly another signal that sets the
idling duty cycle (voltage monitor?)
Martin

 

[Franc Zabkar]

Franc
Pin 1 at 0V (connected to pin 7 Gnd), pin 2 at 5V (connected to pin 14 Vref)
so EA1 is not used. Pin 16 pegged at + supply rail, pin 15 at 4.8V
(connected to pin 3, comp output). I have now deduced that the FETs are
switched hard on because there is an inverter (DS0026) driving them from the
emitters of the output stage of the 494, hence 100% duty cycle. It appears
that this PSU is designed to work 'upside down', i.e. a + feedback input
turns the FETs hard on. The feedback signal is derived from 2 sources, a
current monitor measuring load current (0 because the laser tube doesn't
ignite) and a light monitor signal and possibly another signal that sets the
idling duty cycle (voltage monitor?)
Martin

Is there any chance you could post a hand drawn circuit?


- Franc Zabkar