What is the Difference Between Unregulated and Regulated Power Supplies?

One of the primary functions of a power supply is to convert an input voltage to a desired output voltage. How accurate this voltage is and how much it varies under changing conditions depends on whether the output is regulated and if so, to what degree. When selecting a power supply it is important to understand what regulation is and whether or not it is needed for a particular application.

Background

Regulation is the act of controlling something; in power supplies this typically means controlling the output voltage. To understand its importance and how it works, first consider the circuit in Figure 1.

The circuit in Figure 1 shows a basic linear unregulated dc-dc converter, which works in the following way:

• The ac input voltage is applied across the primary of T1
• The transformer outputs the secondary voltage, V_sec, which is equal to V_ac multiplied by the turns ratio, n (Equation 1)
• The combination of D1 and C_out convert V_sec to a dc voltage, V_dc, equal to the peak of V_sec
• The output voltage, V_out, is then equal to the V_dc minus losses in R_out due to I_out (Equation 2)

The first thing to notice in these equations is that any change to the input voltage will directly affect the output voltage. If R_out is ignored, then V_out is equal to the peak of V_in times the turns ratio. In applications with an input that varies this can lead to large changes in the output voltage. For example, if V_out were 12V with an ac input of 120V and we were to double the input to 240V, V_out would also double to 24V.

In addition to being affected by changes to the input, the load will also affect the output voltage. R_out (which is due to items such as cabling, PCB traces, transformer impedances, etc.) causes a voltage drop between V_dc and V_out that is proportional to the load current. At no load, 0A, V_dc is equal to V_out, but as I_out increases, so does the voltages across R_out, causing V_out to fall. For example, if V_dc were 12V and R_out was 1 ohm, as I_out increased from 0 to 1A the voltage across R_out would increase from 0V to 1V and V_out would fall from 12V to 11V as a result.

The dependencies on input voltage and load conditions, specified in datasheets as line and load regulation respectively, lead to wide variations in output voltage as the conditions change. Some applications may be able to handle this, but many require tighter tolerances under a wide range of conditions. For these applications, regulation is required.

Figure 2 shows a simplified linear regulator which can be added in between the load and R_out of Figure 1, used to regulate the output voltage of Figure 1.

This regulator of Figure 2 works in the following way. V_out is equal to the input voltage minus the voltage drop across the collector and emitter of Q1, V_ce (Equation 3). The op-amp compares V_out to a reference voltage, V_ref, and then amplifies the difference (Equation 4).

This creates a negative feedback loop. Equation 4 shows that if V_out is greater than V_ref, V_base goes negative, turning off Q1 and causing V_ce to increase. By increasing V_ce, V_out is brought down towards the reference voltage. If the voltage were then to fall below the reference voltage, V_base would turn positive and turn Q1 back on, decreasing V_ce and bringing V_out back up. In this way, the regulator is able to maintain a constant V_out under changing line and load conditions.

The linear power supply and regulator were chosen for the previous examples for simplicity, however due to their inefficiencies they are often replaced by more complex switching power supplies. Even with the added complexity in switching power supplies, the fundamental way in which they regulate is the same. The main difference with how they regulate is the control variable. Both linear and switching regulators compare the output to a reference and use this information to control some aspect of the circuit. In the case of the linear regulator, the voltage across a transistor was used to regulate V_out. For many switching regulators the duty ratio (ratio of switch on-time to total switching period) is controlled. In other topologies, such as the resonant LLC, it is the switching frequency that is controlled.

Because the components used to create the feedback loop and references aren’t perfect, neither is the regulation. Datasheets for power supplies, including the unregulated ones, will include some form of information informing the user how much the output voltage can be expected to change under a range of conditions. Sometimes a single number is given as either total regulation or just regulation, which encompasses all conditions. It is also common to see the two listed separately, indicating how much the output will change with respect to a single condition (i.e. Input voltage or Load).

Now, knowing what regulation does and how it works, how do you know which one you need for your application?

Regulated

As discussed earlier, the output of unregulated power supplies is heavily dependent upon the operating conditions. The only way to improve the tolerance on the output is to limit the range of operating conditions. For applications that must accept a wide range of conditions, such as a power supply with a universal input (90 ~ 265 V_ac), and/or those that require a tight tolerance on the output voltage, regulation is required.

Even in applications where the range of conditions is narrow, differences in component tolerances and temperature can lead to differences in output voltage from converter to converter. This is usually specified in datasheets as set-point accuracy. Even if the conditions are constant and the output voltage doesn’t change, without regulation the output voltage may still fall outside of the required tolerance range.

Unregulated

Applications with a narrow range of operating conditions and/or that can accept a wide range of voltages may see some benefit from using an unregulated dc-dc converter. The two primary benefits of an unregulated dc-dc converter compared to that of a regulated converter are size and cost; unregulated converters are often smaller and less expensive than equivalent regulated converters. This is the result of the extra components required to create the feedback loop.

When selecting an unregulated dc-dc converter, the manufacturer will often supply graphs to show the relationship between the output and line and load conditions. The user should inspect these graphs and verify that the voltage is within their limits for all operating conditions. The graph in Figure 3 is one such graph and shows three curves. The min and max lines indicate the set-point accuracy. An individual converter will fall in between these lines with a load line parallel to these curves. The load line shows how much the output voltage can be expected to change as the load goes from min to max.

Conclusion

A tightly controlled voltage is important in many applications. Regulated dc-dc converters can provide tight tolerances over output voltages under a wide range of operating conditions. However, for those applications where a tightly controlled voltage isn’t necessary, it may be beneficial to use an un-regulated dc-dc converter. In these cases, the designer may be able to reduce size and cost by using an un-regulated dc-dc converter.

Categories: Fundamentals , Product Selection

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