Sometimes we want to convert a dc voltage of one value to a dc voltage of another value. For instance, if we have a system with a positive supply of +5 V, we can use a dc-to-dc converter to convert this +5 V to an output of +15 V. Then we would have two supply voltages for our system: +5 and +15 V.
DC-to-dc converters are very efficient. Because they switch transistors on and off, transistor power dissipation is greatly reduced. Typical efficiencies are from 65 to 85 percent.
This section discusses unregulated dc-to-dc converters. The next section is about regulated dc-to-dc converters that use pulse-width modulation. These dc-to-dc converters are usually called switching regulators.
In a typical unregulated dc-to-dc converter, the input dc voltage is applied to a square-wave oscillator. The peak-to-peak value of the square wave is proportional to the input voltage. The square wave is used to drive the primary winding of a transformer, as shown in Fig. 1.
The higher the frequency, the smaller the transformer and filter components. If the frequency is too high, however, it is difficult to produce a square wave with vertical transitions. Usually, the frequency of the square wave is between 10 and 100 kHz.
To improve the efficiency, a special kind of transformer is used in more expensive dc-to-dc converters. The transformer has a toroidal core with a rectangular hysteresis loop. This produces a secondary voltage that is a square wave.
The secondary voltage can then be rectified and filtered to get a dc output voltage. By selecting different turn ratios, we can step the secondary voltage up or down. This way, we can build dc-to-dc converters that step the dc input voltage either up or down.
One common dc-to-dc conversion is +5 to ±15 V. In digital systems, +5 V is a standard supply voltage for most ICs. But linear ICs, like op amps, may require ±15 V. In a case like this, you may find a low-power dc-to-dc converter converting an input +5 V dc to dual outputs of ±15 V dc.
There are many ways to design a dc-to-dc converter, depending on whether bipolar junction or power FETs are used, the switching frequency, whether the input voltage is stepped up or down, and so on.
Dc to Dc Converter Working
Figure 2 shows a design example that uses bipolar junction power transistors. Here is how it works:
- A relaxation oscillator produces a square wave whose frequency is set by R3 and C2. This frequency is in the kilohertz range; a value like 20 kHz would be typical.
- The square wave drives a phase splitter Q1, a circuit that produces two equal-magnitude and out-of-phase square waves. These square waves are the input to Class-B push-pull switching transistors Q2 and Q3.
- Transistor Q2 conducts during one half-cycle, and Q3 during the other half-cycle. The primary current in the transformer is a square wave. This induces a square wave across the secondary winding.
- The square wave of voltage out of the secondary winding drives a bridge rectifier and a capacitor-input filter. Because the signal is a rectified square wave in kilohertz, it is easy to filter.
- The final output is a dc voltage at some level different from the input.
Commercial DC-to-DC Converters
In Fig. 2, notice that the output of the dc-to-dc converter is unregulated. This is typical of inexpensive dc-to-dc converters.
Unregulated dc-to-dc converters are commercially available with efficiencies of about 65 to more than 85 percent. For instance, inexpensive dc-to-dc converters are available for converting +5 to +12 V at 375 mA, +5 to +9 V dc at 200 mA, +12 to ±5 V at 250 mA, and so on.
All of these converters require a fixed input voltage because they do not include voltage regulation. Also, they use switching frequencies between 10 and 100 kHz. Because of this, they include RFI shielding. Some of the units have an MTBF of 200,000 hr. (Note: MTBF stands for “mean time between failure.”)
A switching regulator falls into the general class of dc-to-dc converters because it will convert a dc input voltage to another dc output voltage, either lower or higher.
But the switching regulator also includes voltage regulation, typically pulse-width modulation controlling the on-off time of the transistor. By changing the duty cycle, a switching regulator can hold the output voltage constant under varying line and load conditions.
The Pass Transistor
In a series regulator, the power dissipation of the pass transistor approximately equals the headroom voltage times the load current:
PD = (Vin – Vout)IL
If the headroom voltage equals the output voltage, the efficiency is approximately 50 percent.
For instance, if 10 V is the input to a 7805, the load voltage is 5 V and the efficiency is around 50 percent.
Three-terminal series regulators are very popular because they are easy to use and fill most of our needs when the load power is less than about 10 W.
When the load power is 10 W and the efficiency is 50 percent, the power dissipation of the pass transistor is also 10 W. This represents a lot of wasted power, as well as heat created inside the equipment.
Around load powers of 10 W, heat sinks get very bulky and the temperature of enclosed equipment may rise to objectionable levels.
Switching the Pass Transistor On and Off
The ultimate solution to the problem of low efficiency and high equipment temperature is the switching regulator. With this type of regulator, the pass transistor is switched between cutoff and saturation.
When the transistor is cut off, the power dissipation is virtually zero. When the transistor is saturated, the power dissipation is still very low because VCE(sat) is much less than the headroom voltage in a series regulator.
As mentioned earlier, switching regulators can have efficiencies from about 75 to more than 95 percent. Because of the high efficiency and small size, switching regulators have become widely used.
Switching Regulator Topologies
Topology is a term often used in switching-regulator literature. It is the design technique or fundamental layout of a circuit. Many topologies have evolved for switching regulators because some are better suited to an application than others. Summary Table shows many of the topologies used for switching regulators.
The first three are the most basic. They use the fewest number of parts and can deliver load power up to about 150 W. Because their complexity is low, they are widely used, especially with IC switching regulators.
When transformer isolation is preferred, the flyback and the half-forward topologies can be used for load power up to 150 W.
When the load power is from 150 to 2000 W, the push-pull, half-bridge, and full-bridge topologies are used. Since the last three topologies use more components, the circuit complexity is high.