bd9703 step-down converter

This entry is about power supply based on ROHM Semiconductor’s bd9703 step-down converter.

My experience with this IC started with a failure. First time I tried using this converter resulted in the whole PCB being dead. Shortly after applying power, the power supply started screaming and whistling, as I discovered later – due to horrendous layout, and in some quarter an hour killed most other IC’s on PCB. Such a fail made me dig into the design and layout of this buck converter and make a PCB suitable for evaluating power supplies based on all IC’s in the family: bd9701-bd9703. Finally, I’ve found good layout which I’m planning to use now in all my projects.

The bd9703 is a fully integrated step-down (or buck) converter, similar to lots of other chips present on the market.

It’s main parameters:

Input and output voltages, as well as output current of this chip makes a good replacement for industry standard LM317 series of linear step-down converters, with a benefit of dissipating far less heat (it is switching, not linear). Besides, bd9703 is available in industrial temperature range.

This chip has also two sisters: bd9701 (100 kHz switching frequency) and bd9702 (current up to 3A). Evaluation board described here will suit all those chips, but you’ll have to adjust component values – this is as usual.

Why this chip? It is very popular, used in a plethora of DVD-players, and it makes it available on stock of many component suppliers, even in Russia. Price is also attractive: I’ve bought bd9703 in TO220FP-5 case for $0.78 retail price. But it was before 2011′ Japan earthquake, and now availability may be less and price may be higher – ROHM is Japan company.

Despite popularity and low price tag, one will unlikely find any reasonable documentation for bd9703. There is only a manufacturer’s datasheet, quite messy and incomplete. For example, there are no recommendations on choke and capacitor selection, they give only several criptyc examples. There were also no application notes on the Internet.

Well, you have no docs – will cheat against you. Application schematic is similar to Linear’s famous LT1070 and Simple Switcher series of National Semiconductor (for example, LM2575, LM2586):

Compare it to LM2575 application:

Last year I’ve read through the AN-19 “LT1070 Design Manual” provided by Linear Technology, which describes in great detail the design and calculation of step-down converter – a must-read for everyone. Math is easy to understand, but I was too lazy to compute myself. That’s why I went to National’s website, found similar converter from their portfolio and used their great online tool to calculate choke and capacitor values. Thanks, National Semiconductor! In the end, all step-down converters rely on the same principle and choke, output cap values are calculated with the same formulas.

First version

After initial disaster, I’ve drafted this schematic:

bd9703 evaluation schematic v1

Comments:

Next step was to find good layout. It is very important for a good SMPS layout to trace power and feedback separately.

In this specific case, we have two high-current loops: input loop (C1+C4 – U1.VCC – U1.GND) and output loop (D2 – L1 – C3 – GND, plus U1 to L1 and D2 connection). High-current loops should be made as short as possible. Small-signal nets should be laid out so that high-current nets do not influence them.

Proper ground connection and ground separation is even more important. GND pin of bd9703 is used as both power return (for input and output currents) and signal ground (reference point for feedback). It would be a great mistake to just connect this pin to a ground layer and then connect to this layer all other grounded componets. Current from input and output capacitors to the GND pin will flow through the ground plane in an uncontrollable manner, creating rather unpredictable voltage drop. Ground plane will then have different potentials in different places, and these potentials will depend on the current flow through the high-current loops. Suppose, for example, that grounded pin of R2 will have potential higher than converter’s GND – this will certainly break voltage feedback. Consider also that this voltage will be modulated with the switching current – this will probably introduce positive feedback and unneeded oscillations.

This is my first layout attempt:

Three different traces are connected to the GND(3) pin – one from input loop, another one from the output loop and finally one for feedback loop – this is the small-signal one. The U1-D2-L1-L2-C3-C5 loop is length- and area-minimized. Feedback loop is placed aside from poer loops. Only one net is wierd due to D10 protection diode.

I made this PCB with toner-transfer method, assembled and tested.

Input voltage was 24V, output was 3.3V. I tested with 15 Ohm load and without load. Basically, it works and voltage is fairly constant.

But looking with an ocilloscope I could see big spikes on the output. They appeared with the switching frequency. After an hour of fiddling with capacitors and inductor I finally understtod that it is D10 that breaks everything. Each time the embedded MOSFET of bd9703 closes, current from C1-C2-C4 flows through this diode, and this has several drawbacks:

It happens that this diode stands on the way of switching current and disturbs normal energy transfter from input capacitance to L1-L2. Well, I understand that it is not the description of the underlying physics, but right after I have shorted D10, wierd spikes disappeared from the output. Only normal ripple voltage remained. I have added one more ceramic capacitor in parallel to C4 (same value) and output voltage became even cleaner.

It is clear that low impedance of input capacitors is essential for proper SMPS operation. Ceramic capacitors improve impedance of this group on high frequencies, that’s why they improve converter operation. D10 diode made the opposite – it introduced delay, effectively cutting high frequency response.

Second version

I disassembled v1 after tests and made the second PCB. Here is the schematic:

bd9703 evaluation schematic v2

D10 is removed, and there are 2 ceramic caps on input. Layout looks more elegant:

This PCB is approximately 25×40 mm.

Assembled and tested in the same conditions:

This time everything works great, spikes are gone. I did not measure efficiency, but the converter is cool without heatsink. By the way – this package is entirely plastic, which makes heatsink attachment very easy – you do not need isolate the chip from heatsink.

Third version

After testing the second version, third, more elegant, layout came to my mind (schematic is the same):

But I am lazy already to assemble and test this layout. It looks like everything will work well. If you assemble it – drop me a note.

UPD:  Schematic and PCB for versions 2 and 3 in Diptrace format are here:  ev_bd9703_sch_pcb.zip.

May the Power be with you!

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{ 2 comments to read ... please submit one more! }

  1. Александр

    Добрый день! мне понравилась ваша разводка преобразователя! я хочу попробовать повторить, но рисовать печатку с нуля у меня ещё плохо получается! если вам не сложно, вышлите мне на почту файлы *.lay для эксперементов! в ответ могу выслать свою разводку, она более компактная))

  2. Архив со схемой и платой прицепил к посту в самом низу и выслал вам на почту. Спасибо за внимание к сайту!

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