Viper26k buck converter has an unstable output.

[Elin1480]

We designed a buck converter based on AN5380 with an input of 600Vdc and an output of 12Vdc. 

The device turns on at around 55Vdc, but the output keeps shutting itself off and turning on again (image1). This keeps happening as we turn up the voltage. However, at around 80Vdc, the output changes to many small bursts and then a large spike up to 12Vdc (image 2). Is this only a problem that appears with lower voltages? 

Attached is the schematic of the circuit (image 3). Are the values we chose unsuitable for this application?

[Peter BENSCH]

Welcome @Elin1480, to the community!

Questions about such buck converters have often been discussed here (e.g. here or there), but the necessary conditions are often not considered enough, e.g:

• load current at the output
• coil used (L2 in your case) and its saturation current
• simulation with eDesignSuite

For example, in contrast to the AN5380 mentioned above, you have only used 47µH for the buck coil instead of 1mH, which can have a major impact. In addition, you only used 44nF as input capacitance at the input, which seems very low to me.

Regards
/Peter

[Elin1480]

Thank you for the reply.

I have attempted to use eDesignSuite to simulate the circuit. However, it seems to only work at a maximum of 70V and not at 600V(image). Is this additional circuitry needed for the ic to operate? (image2) With the simulation, I am able to get an output only if those components are added to the input. 

Additionally, are there values you would recommend for the inductor L2 and input capacitance based on a 600V input, 12V output, and a 200ma load? I have attempted using 470uH and 700nf for these two components as well as putting a load on the output. This resulted in a stable output at 65Vdc, but at anything below or above that, the waveform is similar to image2 of the original post with small bursts. 

[Peter BENSCH]

I was referring more to the simulation via the Power Supply Design Tool > AC/DC. Your 600V application can also be interpreted as AC/DC by assuming 425Vac. Of course, the filter that you have in the image2 is then not necessary, but the buffer capacitor (CIN2A/B in your case), at which the voltage is divided with the balancing resistors, is.

I quickly ran this simulation and with 90...425Vacand 0.2A on 390µH and 2x15µF I get a phase margin of 75.24°, which is just below the recommended 76°. With a slightly lower current of 0.17A, 76.06° is also achieved. Please select a variant with at least 0.8A for the coil.

However, the layout also plays a decisive role because the current spikes are very short due to the large voltage difference. So please follow the usual rules for buck regulators meticulously.

[Elin1480]

Hello,

I have tried the design you have suggested as shown in simulation.png but without the AC input circuitry and only the 2 x 15uF capacitors. From 110-120 Vdc in, I am able to get a steady output without any load. Currently, I am getting a rapid square wave at <300 Vdc in (IMG_4883). However, above 300Vdc in, I am getting bursts (IMG_4881). This is all without load. When I try to put load on the output, my output drops to zero and stays there. This happens even in the 110-120 Vdc in range where I have a constant output. 

Is there a certain mode on the chip that is causing this behaviour?

[Elin1480]

Some more information regarding this matter:

The chip that I am using is 265k and not the 267k, which was the one I used for simulation. I have attempted to do the simulation with the 265k but it does not seem to work with the 2x15uF input capacitors.

My capacitance for Cvdd and C4 is higher than in the simulation at 47uF and 2.2uF respectively. I am unsure if that will affect the output of the converter.

[Elin1480]

I was able to get the converter to work below 300v but after 300v, it seems that ton is too short and the output is no longer stable. In the simulation, it also says that the ton is too short, but whenever I try to change components, I am unable to increase the ton. 

[Peter BENSCH]

As with any buck regulator, TON depends directly on the ratio of VIN to VOUT. So if VIN increases with constant VOUT, TON must inevitably decrease. As mentioned above, this and similar problems occur with flyback regulators that are forced to buck, because the voltage difference is usually too large to deliver the desired output power in a stable manner.

[Elin1480]

So since I want an input of ~600Vdc, the only way I can increase the TON is by increasing the output voltage? Would I also be able to increase it by changing the amount of load I have on the output?