Isolated DC/DC converters are essential components for a wide range of applications including energy metering, PLC, IGBT driver power supplies, industrial fieldbus and industrial automation. These converters are often used to provide galvanic isolation, improve safety, and improve noise immunity. Moreover, they can also be used to generate multiple output voltage rails including bipolar power rails
According to the output voltage regulation accuracy, isolated DC/DC converters are often divided into three categories, namely: regulated, unregulated and semi-regulated. This article will discuss various adjustment schemes and corresponding topologies. The factors affecting the accuracy of the adjustment were examined in detail. This will create some design tips that can improve the adjustment accuracy in the actual design. In addition, the pros and cons of each option are described and are intended to guide the selection of a suitable solution for a specific application need
Feedback and control of isolated DC/DC converters. The
Isolated DC/DC converter typically uses a transformer to electrically isolate the output and power stage inputs (Figure 1)
Figure 1: Block diagram of the isolated DC/DC converter power stage.
In a closed-loop isolated DC/DC converter (Figure 2), the feedback circuit is responsible for detecting the output voltage and generating an error by comparing the sensed voltage to its target value (feedback voltage reference). This error is then used to adjust the control variable (in this case the duty cycle) to compensate for the output deviation. In addition, galvanic isolation between the primary side and the control circuitry on the secondary side is also essential. This type of isolation can be achieved by using a transformer or optocoupler. Assuming that the reference voltage VREF remains accurate and stable over the entire temperature range, the adjustment accuracy will depend primarily on the accuracy of the output voltage detection (in other words, the similarity between VSENSE and VOUT)
Figure 2: Feedback and control of a closed-loop isolated DC/DC converter
Unregulated isolated DC/DC converter.
Unregulated isolated DC/DC converters (also known as “open-loop isolated DC/DC converters”) are widely used in applications that do not require precise output voltages. A typical application is a push-pull converter with a 50% fixed duty cycle (Figure 3). The control circuit includes only one oscillator and two gate drivers that generate two complementary gate signals with a 50% fixed duty cycle to drive Q1 and Q2. Select the appropriate transformer turns ratio to provide the desired output voltage. No feedback circuitry or signal isolators are required, reducing cost and solution size
Figure 3: Unregulated push-pull converter with 50% fixed duty cycle.The
push-pull converter is essentially a forward-derived topology. When operating at a fixed duty cycle of 50%, the output voltage regulation can be elaborated using the equivalent circuit in Figure 4. R is the equivalent resistance of the secondary transformer windings and traces. The output voltage can be expressed by equation (1): The VR in
Is the voltage drop across resistor R, and VF is the diode forward voltage drop, both of which are related to the load current. Moreover, VR and VF will also change with ambient temperature, as does VOUT. As shown in equation (1), in addition to the load current and ambient temperature, VIN is also a factor affecting VOUT. These factors are not compensated at all and may result in significant output voltage changes. This is why the converter is “unregulated”
Figure 4: Equivalent circuit of an unregulated push-pull converter
Similar to push-pull converters, other common topologies for unregulated isolated DC/DC converters are half-bridge and full-bridge (H-bridge) converters. These unregulated isolated DC/DC converters are often used as DC transformers to provide galvanic isolation due to their low cost and very simple circuitry. Low dropout (LDO) regulators are commonly used as post regulators to provide low noise and low ripple power supplies
Adjusted Isolated DC/DC Converter
In an unregulated isolated DC/DC converter, the input voltage, load current, and ambient temperature all affect the output voltage accuracy. This is unacceptable in applications where precise output voltages and tight regulation are critical, so a regulated isolated DC/DC converter should be used. Let us take the flyback converter shown in Figure 5 as an example to elaborate how to achieve strict adjustment. The regulated flyback converter has an additional feedback circuit compared to an unregulated push-pull converter (Figure 3). In addition, an optocoupler is used to transfer control signals from the secondary side to the primary side while galvanic isolation is achieved
The advantage of using an optocoupler is that the feedback circuit can be placed on the secondary side. This allows the output voltage to be sensed and regulated directly (ie, VSENSE=VOUT), which in turn compensates for all effects of input voltage, load current, and temperature on output voltage regulation. Therefore, it is generally expected to achieve a tight adjustment accuracy of 1% to 3% over the entire operating input voltage, load current, and temperature range
There are several disadvantages to using optocouplers. First, the optocoupler introduces an extra pole in the control loop, which reduces the converter bandwidth. Second, optocouplers have large “unit-to-unit variations” and temperature and lifetime degradation in current transfer ratio (CTR), thus constraining the control loop design
Figure 5: Adjusted flyback converter with an optocoupler
Semi-regulated isolated DC/DC converter. The
Unregulated isolated DC/DC converter does not require any optocouplers, but it does not provide any adjustments. In contrast, a regulated isolated DC/DC converter provides tight output voltage regulation, yet requires an optocoupler. In many applications, customers may not want to use optocouplers, but require some degree of adjustment of the output voltage. The so-called “semi-regulated” isolated DC/DC converter will be the right solution
From the perspective of output voltage regulation, the semi-regulated isolated DC/DC converter is between the unregulated and regulated isolated DC/DC converters. Similar to the regulated isolated DC/DC converter, the semi-regulated isolated DC/DC converter also has a feedback circuit. However, it does not directly sense and adjust the output. Instead, it only detects a voltage similar to the output voltage on the secondary side, but typically referenced to the primary input voltage. These methods may not achieve the same output voltage as the tuned isolated DC/DC converter, but they eliminate the optocoupler and achieve fairly good output voltage regulation. The three examples discussed in this article are Fly-Buck converters, flyback converters with cross-regulated outputs, and primary side regulation (PSR) flyback converters
1. Fly-Buck Converter. The
Fly-Buck converter is basically a synchronous buck converter with an additional winding coupled to its inductor to generate an isolated output (VOUT). In addition to the isolated output on the secondary side, the Fly-Buck converter also provides a regulated output (VP) on the primary side. The primary side output is adjusted in the same way as a stand-alone synchronous buck converter, as in equation (2): D in the formula
Is the duty ratio of the buck switch Q1 in Fig. 6. When the low side synchronous switch Q2 is turned on, VP is reflected to the secondary side and rectified to VOUT. The equivalent circuit is shown in Figure 7. VOUT can be calculated using equation (3):
Figure 6: Fly-Buck converter
Figure 7: Equivalent circuit of the Fly-Buck converter.
Similar to the unregulated push-pull converter described in equation (1) and Figure 4, the isolated output of the Fly-Buck is a function of VR and VF, both of which depend on the load current and temperature. However, VP is a constant voltage that is regulated by the feedback circuit, which makes VP (and therefore VOUT) independent of VIN. For the isolated output of the Fly-Buck converter, the effect of VIN is compensated, but the effects of load current and temperature are not compensated. Thus, the Fly-Buck converter is classified as a semi-regulated isolated DC/DC converter
When Q1 is turned on, the output capacitor COUT is discharged to provide a load current. When Q2 is turned on, the output capacitor charge is supplemented to maintain regulation. In fact, the transformer has more or less leakage inductance, which determines the ramp rate of the current in the secondary winding used to charge the output capacitor. Leakage inductance and duty cycle affect output voltage regulation. Leakage inductance should be minimized and the maximum duty cycle should be carefully chosen to mitigate their effects on regulation. With the right design, it is possible to achieve an output voltage regulation of 5% to 10% (depending on the load current range)
2. Flyback converter with cross-regulated output. The
Flyback converter makes it easy to generate multiple outputs without having to add additional output filter inductors like other DC/DC converter topologies. In a multiple output configuration (Figure 8), only one output, Vaux, is directly regulated, while the other VOUTs rely on cross regulation. In general, the optocoupler of the regulated flyback converter shown in Figure 5 can be eliminated by having the regulated output Vaux referenced to the input VIN on the primary side. The isolated output VOUT on the secondary side can be given by equation (4):
Figure 8: Flyback converter with cross-regulated output.VRs and VRa in
Are the equivalent resistance voltage drops of the secondary winding and the auxiliary winding, respectively. VRs, VRa, VFD1, and VFD2 are all functions of their own current. The current flowing in the secondary winding and the auxiliary winding is non-uniform, thus causing a mismatch in load regulation between VOUT and Vaux. As a result, the load regulation of VOUT is not as good as Vaux. The isolated output is independent of VIN, which indicates excellent line input voltage regulation. Since the cross regulation output depends on the load current range, an output voltage adjustment of 5% to 10% can usually be achieved
3. PSR flyback converter.
Although the line input voltage regulation performance is excellent, Fly-Buck and flyback converters that rely on cross regulation cannot compensate for the effect of load current on output voltage regulation. Therefore, the accuracy of the output voltage depends on the load current. The PSR flyback converter (Figure 9) is designed to minimize this dependency by more accurately detecting the output voltage
Figure 9: PSR flyback converter
By operating in discontinuous conduction mode (DCM) or boundary conduction mode (BCM), the secondary current returns to zero during each switching cycle. Figure 10 shows the auxiliary winding voltage distribution in the DCM. The PSR flyback converter detects the auxiliary winding voltage VSENSE at a corner point (where the secondary current is approximately zero) through a dedicated discriminator and sampler circuit. At the sampling point, there is no resistance drop across the windings and traces because the secondary current is zero. Moreover, the forward voltage drop across the diode at the sample point becomes a constant VOFFSET, independent of the actual load current. Thus, the detection voltage becomes:
Figure 10: Voltage detection scheme for PSR flyback converter
Because of this, regardless of the load current, VSENSE is a good representation of the output voltage and has only one fixed voltage that can be offset by adjusting the voltage feedback resistor divider. In this way, the effect of the load current on the output voltage regulation is minimized and good load regulation is expected. Since the PSR flyback converter compensates for line input voltage and load variations, it is possible to achieve a total regulation performance better than 5%
In conclusion.
For the purpose of galvanic isolation and safety and improved noise immunity, the secondary side and the primary side are electrically isolated in an isolated DC/DC converter. This isolation is used by both the power stage and the control circuitry. The detection and regulation of the output voltage determines the accuracy of the output voltage regulation. The unregulated isolated DC/DC converter has the lowest cost and the simplest circuit, but no regulation. The regulated isolated DC/DC converter provides tight regulation over the entire line input voltage, load and temperature range, but requires the use of an optocoupler or digital isolator IC. Semi-regulated isolated DC/DC converters compromise between output voltage regulation and circuit complexity. The most appropriate solution should be chosen to accord to the specific application needs
From EDN Electronic Technology Design