DC Power Supply Design Using Buck-Boost Converters

Key Takeaways

• Usually, a non-isolated DC power supply design uses classical buck-boost converters, SEPIC, Cuk, Zeta, or Luo converters to step up or step down the voltage.

• Basic buck-boost converters supply a negative-polarity output with respect to the common terminal of the input DC voltage. The output voltage can be greater than or less than the input voltage. 

• The turning on and off of the switch with a duty ratio below 0.5 gives an output voltage less than the input DC source. 

In portable electronics and automobile electronic devices, DC power supply design using buck-boost converters is preferred

In portable electronics and automobile electronic devices, DC power supply design using buck-boost converters is preferred due to the large output voltage variations of the battery source. Buck-boost converters process the varying voltages from the battery and bring the desired voltage to greater than or less than the average battery voltage. Usually, a non-isolated DC power supply design uses classical buck-boost converters, SEPIC, Cuk, Zeta, or Luo converters to step up or step down the voltage. In this article, we will discuss buck-boost converter-based DC power supplies.  

The Use of Buck-Boost Converters in DC Power Supply Design 

Consider working on an electronics circuit where the DC supply is drawn from a battery of 12 V and the circuit requirement is 15 V. In such applications, you can use boost converters to step up from 12 V to 15 V. If you want to supply 8 V from the same battery to another circuit, using a buck converter is the best option. With a buck converter, the 12 V battery voltage can be stepped down to 8 V. DC power supply design to the first and second circuit is based on a boost converter and buck converter, respectively.

Imagine a circuit that requires 8 V and 12 V alternatively under different load conditions. For such applications, instead of using buck and boost converters, you can combine the two voltage conversion actions into one circuit, called a buck-boost converter. The incorporation of a buck-boost converter into a DC power supply design helps provide voltage over a range that can be either less than or greater than the average DC input voltage.

Classic Buck-Boost Converter Operations 

The figure above shows a buck-boost converter circuit diagram, where the converter circuit supplies a negative-polarity output with respect to the common terminal of the input DC voltage. There are two operating states: open switch (switch turned off) and closed switch (switch turned on). When the switch is turned on, the inductor charging the diode is reverse-biased, and the capacitor supplies the output voltage. When the switch is turned off, the inductor discharges to the connected load. The duration for which the switch is turned on and off influences buck-boost converter operations. The transfer function of a buck-boost converter can be given by the equation:

In this equation, D is the duty ratio of the switch. The turning on and off of the switch with a duty ratio below 0.5 gives an output voltage less than the input DC source. A duty ratio above 0.5 boosts the input DC voltage, so the converters offer higher output voltage.

Buck-boost converter operations can be summarized with the following table.

DC Power Supply Design Using Buck-Boost Converters

Let’s talk through the design of a DC power supply based on a buck-boost converter. Imagine the input DC voltage is 15 V and the buck-boost converter must supply 10 V to a load of 10 Ω. The switching frequency (f) selected is 20 kHz. The duty cycle can be calculated using the equation:

Let the buck-boost converter operate at critical conduction mode (CRM), otherwise the converter is operating at the boundary condition between the continuous conduction (CCM) mode and the discontinuous conduction mode (DCM).

The inductor value for CRM operation can be given by the equation:

The capacitor can be selected based on the output voltage ripple value. Usually, the ripple content is selected as less than 1% of the output voltage and can be given by the equation:

The inductor and capacitor values are calculated as 90µH and 2mF, respectively, for the given circuit specification.

We have discussed a basic buck-boost converter that gives a negative polarity voltage; let’s take a look at other buck-boost converter topologies next.

Other Buck-Boost Converter Topologies

Non-isolated buck-boost converter topologies are Cuk converters, single-ended primary-inductor converters (SEPIC), Zeta converters, Luo converters, and flyback (isolated) converters. Each circuit topology has strengths and weaknesses, and the appropriate one for a given application is selected depending on the size, power, efficiency, and isolation requirements.

DC power supply design using a buck-boost converter can be with either an isolated or non-isolated circuit. Like the performance achieved from buck-boost topologies, the components and sizes of DC supply boards using buck-boost converters also vary.

Cadence’s suite of design and analysis tools can help you  design power supply boards. Our tools offer features for designing the power circuit and control circuit of buck-boost converter-based power supplies.

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