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Chinese “1200W” DC-DC Boost Converter DOA Fix

1200W DC-DC Converter
1200W DC-DC Converter

Ah the curse of the Chinese Electronics strikes again. These large DC-DC boost converters have become very common on the likes of AliExpress & eBay, and this time my order has arrived DOA… On applying power, the output LED lights up dimly, and no matter how I twiddle the adjustment pots, the output never rises above the input voltage.

Boost Converter Topology
Boost Converter Topology

From the usual topology above, we can assume that the switching converter isn’t working, so the input voltage is just being directly fed through to the output. The switching IC on these converters is a TL494,

Control Circuitry
Control Circuitry

The switching IC on these converters is a TL494,with it’s surrounding support components, including a LM358 dual Op-Amp. Power for this lot is supplied from the input via a small DC-DC converter controlled by an XL Semi XL7001 Buck Converter IC. Some testing revealed that power was getting to the XL7001, but the output to the switching controller was at zero volts.

Inductor
Inductor

The 100ยตH inductor for this buck converter is hidden behind the output electrolytic, and a quick prod with a multimeter revealed this inductor to be open circuit. That would certainly explain the no-output situation. Luckily I had an old converter that was burned out. (Don’t try to pull anything near their manufacturer “rating” from these units – it’s utter lies, more about this below).

Donor Converter
Donor Converter

The good inductor from this donor unit has been desoldered here, it’s supposed to be L2. This one had a heatsink siliconed to the top of the TL494 PWM IC, presumably for cooling, so this was peeled off to give some access.
After this inductor was grafted into place on the dead converter, everything sprang to life as normal. I fail to see how this issue wouldn’t have been caught during manufacture, but they’re probably not even testing them before shipping to the distributor.
The sensational ratings are also utter crap – they quote 1.2kW max power, which at 12v input would be 100A. Their max input rating is given as 20A, so 240W max input power. Pulling this level of power from such a cheaply designed converter isn’t going to be reliably possible, the input terminals aren’t even rated to anywhere near 20A, so these would be the first to melt, swiftly followed by everything else. Some of these units come with a fan fitted from the factory, but these are as cheaply made as possible, with bearings made of cheese. As a result they seize solid within a couple of days of use.
Proper converters from companies like TDK-Lambda or muRata rated for these power levels are huge, with BOLTS for terminals, but they’re considerably more expensive. These Chinese units are handy though, as long as they are run at a power level that’s realistic.

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SEPIC Regulator Test Rig

Regulator Hookup
Regulator Hookup

Here’s the regulator hooked up with test clips, on the right is the supply from the ultracapacitor bank, while on the left is the output, feeding a 2.3A brushless fan as a test load.

These regulators do get warm, even with no load, with a 2.3A load on the output, the temperature stays warm to the touch.

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ICL Barcode Scanner

Top
Top

An ICL barcode scanner from the 80s is shown here. This is the top of the unit with cover on.

Cover Removed
Cover Removed

Plastic cover removed from the unit showing internal components. Main PSU on left, scan assembly in center. Laser PSU & Cooling fan on right. Laser tube at top.

Scan Motor
Scan Motor

Closeup of laser scan motor. This unit scans the laser beam rapidly across the glass plate to read the barcode.

Controller PCB
Controller PCB

View of the bottom of the unit, showing the controller PCB in the centre.

Scan Motor Driver
Scan Motor Driver

The 3-phase motor driver circuit for the scan motor. 15v DC powered.

Laser Unit
Laser Unit

This is the laser unit disconnected from the back of the scanner. HT PSU is on right hand side, beam emerges from optics on left.

Laser Unit Label
Laser Unit Label

This unit is date stamped 1987. The oldest laser unit i own.

Tube PSU
Tube PSU

Laser tube power supply. Input voltage: 24v DC. Output: 1.8kV 4mA.

Laser PSU Board
Laser PSU Board

Rear of HT PSU. Obviously the factory made a mistake or two ๐Ÿ™‚

Laser Tube Mounting
Laser Tube Mounting

Top cover removed from the laser unit here shows the 1mW He-Ne tube. Manufactured by Aerotech.

Tube Label
AeroTech He-Ne Tube

Tube label. Manufactured July 1993. Model LT06XR.

Plasma
Plasma

Here the tube has been removed from it’s mount to show the bore down the centre while energized.

OC Mirror
OC Mirror

OC end of the tube shown here lasing.

Beam
Beam

Beam output from the optics on the laser unit.

Tube Optics
Tube Optics

Optics built into the laser unit. Simple turning mirror on adjustable mount & collimating lens assembly.

Scan Lines
Scan Lines

Kind of hard to see but the unit is running here & projecting the scan lines on the top glass.

Laser Tube Mounting
Laser Tube Mounting

Laser tube mounting. A combo of spring clips & hot glue hold this He-Ne tube in place