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trouble_with_Foldback.php    8433 Bytes    28-03-2019 18:59:52

Trouble with that Foldback Current Limiting Thing

Installed in e.g. IHB200-0.12-A (Bel Power) or LNB200-0.12 (XP)

Foldback Current Limiting is a technic used to economise on heatsink. Most users have problems getting those devices started, as their load has a high inrush current. So did we.

Foldback Current Limiting

✈ What is Foldback Current Limiting ?

Foldback is a current limiting feature (we'd prefer bug) of power supplies and power amplifiers. When the load attempts to draw overcurrent from the supply, foldback reduces both the output voltage and current to well below the normal operating limits. Under a short circuit, where the output voltage has reduced to zero, the current is typically limited to a very small fraction of the maximum current. Says Wikipedia.

With that behaviour, the dissipated power of the linear voltage regulating transistor can be reduced. Also the size of the heatsink can be reduced.

UA723 Foldback Current Limiting
Foldback Current Limiting courtesy of TI (UA723)
Laboratory Power Supply Current Limiting
Laboratory Power Supply, courtesy of Saure

✈ Why should I care ?

The trouble shows up, when you want to power a circuit which has large capacitors in the supply line. As we know, capacitors draw a lot of current, when switched on. This adds up to the current your circuit draws under normal conditions (which may be well in the safe operating area of the power supply). This sum of currents may cause the power supply to fall into Foldback Current Limiting. And yes - it stays there - unless you switch it off again.

✈ Our Quick-Hack

The solution is to dis-enable the current sensing during start-up. This is done by placing a capacitor over the current sensing unit (UA723 in our case). The capacitor will present zero volts across the sensing unit and therefore allow the power supply to start. When done, the capacitor will be charged to a voltage representing the current. For the used unit here (HB200-0.12-A) we placed a 1000 µF, 16 V capacitor over the current sensing unit. This may vary according to your load. Use it as a starting value for optimisation. The downloads below show a schematic of the LNB200-0.12 as we saw it. And yes, this Quick-Hack worked fine during our tests, but we were uncomfortable using them as a long term solution. For the lawyers out there : DO NOT DO THIS AT HOME (OR AT THE OFFICE). We shall not be held responsible for anything.

Regulation is done by good old 723

✈ Downloads (Schematics of LNB200-0.12)

✈ The Real Deal

Is a solution without that foldback thing. Invest some money in a larger heatsink, do the math for the maximum power dissipation, and you may end up with something like our PETH-200V.

The design is similiar to our QO-408 (a replacement for the VB408), but uses more widely available components and - of course - a big heatsink.

You will also need an isolation transformer (In 230V, Out 230V or 2 x 115V), but they are widely available at low cost (100 W - 45 €, 200 W - 63 €, 300 W - 70 €, seen at e.g. Reichelt in March 2019).

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t1 = 6585 d

t2 = 229 ms

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