DC Power Supply Design

Regulated and unregulated power supplies are the main classifications of DC power supplies. Regulated power supplies are further divided into two types: linear regulated and switch mode. Switching mode power supplies can be either primary or secondary switching mode.

The following list describes each DC power supply type and design.

1. Unregulated Power Supplies

Unregulated power supplies use the AC mains as the input. The AC voltage is first passed through a step-down transformer. The lower secondary voltage is then rectified and converted to the corresponding direct current flow. The output voltage of the rectifier is then smoothed by a capacitor. As the name implies, an unregulated power supply does not have a voltage regulator as part of the circuit. In fact, any change in the main line directly affects the output.

Unregulated power supplies are simple in design and durable, with a typical efficiency of about 80%.

The primary use of unregulated power supplies is for electromechanical applications, which do not require a defined output voltage, such as powering contactors.

Key benefits include.

✱ High efficiency

✱ Heavy duty

✱ Cost-effective

Disadvantages include.

✱Bulk size

✱High residual ripple

✱No DC power (input)

2. Linear Regulated Power Supplies

Linear regulated power supplies use the AC-DC conversion process discussed in Chapter 3.

The AC supply voltage is stepped down to a lower level using a transformer, and rectified and filtered. The final step involves smoothing the regulation of the DC voltage, usually using a power transistor, to keep the output at a constant value.

For linear regulated power supplies, the power transistor acts as a variable transistor.

As it passes through the power transistor, there is a high loss of energy; this energy is given off as heat. Therefore, the power supply needs to be properly ventilated.

Due to these losses, the efficiency of linear regulated power supplies is typically about 50%.

High-precision medical devices require very precise output voltages. For this application, linear regulated power supplies are often used.

Key benefits include.

✱ Short regulation time

✱ Low residual ripple

✱ Simple circuitry

Disadvantages include.

✱Low efficiency

✱ Large code

✱No DC power (input)

3. Primary switch-mode power supply

The primary switch mode power supply uses rectification, filtering and chopping/switching of the AC mains in the first few steps.

When the DC voltage is chopped, it only means that it is switched periodically at 40-200 kHz using power transistors.

While power transistors are used as variable transistors in linear regulated power supplies, they are used as switches in the main switch mode power supply.

A square wave AC voltage is generated in the chopper/switching step, which is then used as the input to the high frequency transformer in the secondary circuit. The voltage is then rectified and smoothed again.

Depending on the load, the chopping rate can be varied to control the amount of energy converted to the secondary circuit.

Because high frequency AC voltages are used, the primary switch mode power supply can use transformers that are typically smaller than those required for low frequency conversion.

Primary switch-mode power supplies can use a wide range of input voltages. DC voltages can also be used as inputs. This is because the input voltage does not directly affect the output voltage.

Short-time buffering may also be up to 200 ms, which is essential in the event of a supply voltage failure.

However, it is important to note that the supply buffer failure time is limited by the size of the capacitor.

Larger capacitor sizes can provide higher capacity and longer buffering times, but this is not desirable in smaller power supplies. Therefore, the power supply should be optimized to have "just the right" buffer time and capacitor size.

Primary switch-mode power supplies are widely used in electronic and electromechanical applications.

Key benefits include.

✱ Small size

✱ Lightweight

✱ Wide input voltage range

✱Easy regulation

✱High efficiency

✱DC power supply

✱Buffering in case of supply voltage breakdown

Disadvantages include.

✱Complicated circuitry

✱Power supply contamination

✱High frequency requires interference suppression

✱Expensive

4. Secondary Switching Mode Power Supplies

A secondary switch-mode power supply is very similar to a primary switch-mode power supply, but the chopping is done on the secondary side.

In fact, a larger transformer is required to convert the 50/60 Hz supply voltage.

However, since the transformer also acts as a filter, power supply contamination can be reduced.

The main advantages include.

✱ High efficiency

✱Easy to adjust

✱ Wide input voltage range

✱ Low power supply contamination

Disadvantages include.

✱ Bulk size

✱No DC power (input)

✱Expensive

In industrial applications, primary switching power supplies are the most widely used type due to their wide input voltage range, high efficiency, and small size.

Efficiency, regulation time, weight and size, residual ripple, cost and application area are the most important factors to consider when selecting the right power supply for a new engineering application and upgrading an existing unit.