How to control the non-isolated bidirectional buck-boost converter?

Question

I have been working on a control software for below given bidirectional buck-boost converter.

The planned usage is for charging and discharging of a traction battery in electrical vehicle. I am not sure how to control this type of dc-dc converter.

From my perspective there four operating modes:

a) battery charging in buck mode (S1: pwm, S2: off, S3: off, S4: off)

b) battery charging in boost mode (S1: on, S2: off, S3: off, S4: pwm)

c) battery discharging in boost mode (S1: off, S2: pwm, S3: on, S4: off)

d) battery discharging in buck mode (S1: off, S2: off, S3: pwm, S4: off)

As far as the operating mode selection I think that

a) selection between battery charging and discharging takes place based on commands comming from higher layer which is able to made a decision whether there is enough energy in the system or there is a lack of energy

b) selection between buck mode and boost mode takes place based on relation between Vbus1 and Vbus2 voltages

As far as measurement both Vbus1 and Vbus2 are measured along with the inductor current. Only for clarification. The bus1 consists of capacitor C1, the bus2 consists of capacitor C2 in parallel to the traction battery.

As far as the control algorithm I think that there should be two cascaded PI loops (external voltage loop and internal current loop). At the output of the internal PI loop there is the duty cycle for the transistors switching. One set of cascaded PI loops is for battery charging (battery voltage - Vbus2 - is regulated) and one set of cascaded PI loops is for battery discharging (dc bus voltage - Vbus1 - is regulated).

Please can anybody tell me wheter my understanding how to cotrol the bidirectional buck-boost converter is correct? If not so can anybody recommend me some information source? Thanks in advance for any suggestions.

Answer 1

Battery charging in buck mode

battery charging in buck mode (S1: pwm, S2: off, S3: off, S4: off)

No, it's like this: -

In other words it's a synchronous buck regulator as per this: -

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Battery charging in boost mode

battery charging in boost mode (S1: on, S2: off, S3: off, S4: pwm)

Nearly right (you have assumed a non-synchronous boost circuit). A synchronous boost (more efficient) is like this: -

In other words it's a synchronous boost charger like this: -

Given that its bidirectional there's no need to repeat myself for the other two modes.

Answer 2

Your diagram is missing any way to sense, and therefore control, current. As drawn, it a voltage source, which is not good, strung as it is between two low impedance batteries.

While which battery gets charged would be determined by energy state of the batteries, and which mode of buck or boost would be selected by which side has higher voltage, the actual charge process then ought to instruct the charger to send X amps from donor to recipient battery. This cannot be done.

In order to protect that H bridge, given that all components shown are low impedance, as a minimum the current sense ought to be in series with the inductor, to directly control the timing of the switches, and make the bridge a current source rather than a voltage source.

You could relate the inductor current to the charging current by dead reckoning, knowing the battery voltages, as no great precision is required. You could put current sense resistors into one or both supply lines as well if you wanted to sense the charge current directly.