Understanding isolated DC/DC converter voltage regulation
Regulated isolated DC/DC converters
Input voltage, load current, and ambient temperature all have an impact on the output voltage accuracy in an unregulated isolated DC/DC converter. This is not acceptable to applications where a precise output voltage and tight regulation are critical, and a regulated isolated DC/DC converter should be adopted. A flyback converter in Figure 5 is taken as an example to elaborate how the tight regulation is achieved. Compared with the unregulated push-pull converter (Figure 3), the regulated flyback converter has an additional feedback circuitry. Also an optocoupler is used to transfer the control signal from the secondary side to the primary side while achieving galvanic isolation.
The advantage of using an optocoupler is that the feedback circuitry can be placed on the secondary side. As such, the output voltage can be directly sensed and regulated (that is VSENSE=VOUT), which in turn compensates all the effects of input voltage, load current and temperature on output voltage regulation. As a result, a tight regulation in the range of ±1% to ±3% usually can be expected over all operating input voltage, load current, and temperature conditions.
There are several disadvantages to use an optocoupler. First of all, an optocoupler introduces an extra pole in the control loop, which reduces the converter’s bandwidth. Second, an optocoupler has large unit-to-unit variation and temperature and lifetime degradation in the current transfer ratio (CTR), which imposes constraints on the control loop design.
Figure 5. Regulated flyback converter using an optocoupler
Semi-regulated isolated DC/DC converters
Unregulated isolated DC/DC converters do not require any optocoupler, but fail to provide any regulation. Conversely, regulated isolated DC/DC converters provide tight output voltage regulation, but require an optocoupler. There are many applications where the customer may not want an optocoupler, but require the output voltage to be regulated to some extent. The so-called semi-regulated isolated DC/DC converter will be the appropriate solution.
From an output voltage regulation perspective, the semi-regulated isolated DC/DC converter is something between the unregulated and regulated isolated DC/DC converters. Like a regulated isolated DC/DC converter, the semi-regulated isolated DC/DC converter also has a feedback circuit. However, it does not sense and regulate the output directly. Instead, it just senses a voltage, which resembles the output voltage on secondary side but is commonly referenced with the primary input voltage. These techniques might not be able to achieve as accurate an output voltage, but they eliminate the optocoupler while achieving decent output voltage regulation. Three examples discussed in this article are Fly-Buck converter, flyback converter with cross-regulated output, and the primary side regulation (PSR) flyback converter.Fly-Buck converter
A fly-buck converter (Figure 6) 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 secondary side, Fly-Buck converters provide a regulated output (VP) on the primary side. The primary side output is regulated in the same way as a standalone synchronous buck converter (2):
where D is the duty cycle of the buck switch Q1 in Figure 6. When the low-side synchronous switch Q2 conducts, VP is reflected to secondary and rectified as VOUT. The equivalent circuit is shown in Figure 7. VOUT can be calculated by (3):
Like the unregulated push-pull converter described by equation 1 and Figure 4, the isolated output of the Fly-Buck is a function of VR and VF, which are both load-current and temperature-dependent. However, VP is a constant voltage regulated by a feedback circuit, which makes VP and, thus, VOUT independent of VIN. To the isolated output of a Fly-Buck converter, the effect of VIN is compensated, but the effects of load current and temperature are not compensated. So the Fly-BuckTM converter falls into the semi-regulated isolated DC/DC converter category.
Figure 6. Fly-Buck converter
Figure 7. The equivalent circuit of a Fly-Buck converter
When Q1 is on, the output capacitor COUT is discharged, supplying the load current. When Q2 is on, the output capacitor charge is replenished to maintain regulation. In practice, the transformer has more or less leakage inductance, which determines ramp rate of the current in the secondary winding to charge the output capacitor. The leakage inductance along with duty cycle impacts on the output voltage regulation. The leakage inductance should be minimized and the maximum operating duty cycle should be chosen carefully to mitigate their impacts on the regulation. By proper design, roughly a ±5 to ±10 percent output voltage regulation could be achieved, depending on the load current range.