DC-DC Bidirectional Buck boost converter

General
1

Hello everyone,
I am working on a bidirectional buck-boost DC-DC converter simulation in PLECS, stepping between a 49 V battery and a 400 V DC link without a transformer.
My control scheme uses a cascaded PI controller structure:

  • The outer loop regulates the DC link voltage (target 400 V) by comparing the measured DC link voltage with a 400 V reference.
  • The inner loop regulates the battery current by comparing the battery current with the output of the outer voltage loop.
  • The PI controller outputs are fed through a saturation block (0.05 to 0.95) and then into a sawtooth PWM generator to drive the IGBTs.

However, despite this setup, I am only seeing about 50 V on the DC link side in boost mode instead of the expected 400 V.

Could you please help me understand:

  1. What could be causing the DC link voltage to stay low despite the control structure?
  2. Are there recommended starting values for the PI controller gains (Kp and Ki) for this kind of high boost ratio converter?
  3. Any suggestions on improving the control strategy or simulation setup to achieve a stable 400 V output?

For reference, my inductor value is 400 μH, the switching frequency is around 20 kHz, and the load is modelled as a 55 Ω resistor on the DC link side.

Thank you in advance for your help!

This is the plecs file
buckboost.plecs (57.2 KB)

2
  1. What could be causing the DC link voltage to stay low despite the control structure?

Your simulation cannot electrically reach your desired operating point. The reason is the internal resistance of your battery. Even without the outer control structure, just fixing the duty cycle, you’ll see that the desired output voltage cannot be reached. Your 55 Ohm at 400V at the output would be ~60A from the battery. That ~60A flows through 1.1 ohms of resistance (i.e. it will not converge at your desired

  1. Are there recommended starting values for the PI controller gains (Kp and Ki) for this kind of high boost ratio converter?

There are several boost converters in the PLECS demo model library. See " Single-Phase PV Inverter with Partial Shading" or the “Boosted Motor Drive” example which have a similar control structure. The model initialization commands calculate the gain values using an approach called Symmetric Optimum and Magnitude Optimum, which is documented in the “Three-Level Boost PFC Converter” demo.

  1. Any suggestions on improving the control strategy or simulation setup to achieve a stable 400 V output?

Start with improving and understanding your electrical model. Then work sequentially by tuning the inner current loop (fixed output voltage) and outer voltage loop.

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