“Another approach is an isolated buck or fast buck DC/DC converter, a synchronous buck converter with coupled Inductor windings. According to Texas Instruments’ application report 1, “Designing an Isolated Buck (Fly-buck) Converter,” fly-bucks are a better choice for low-power isolated outputs, especially when power levels are below 15. In fact, TI’s applications Reports indicate that its Fly-buck is a cost-effective solution for generating multiple output bias supplies below 15 W.
Another approach is an isolated buck or fast buck DC/DC converter, a synchronous buck converter with coupled inductor windings. According to Texas Instruments’ application report 1, “Designing an Isolated Buck (Fly-buck) Converter,” fly-bucks are a better choice for low-power isolated outputs, especially when power levels are below 15. In fact, TI’s applications Reports indicate that its Fly-buck is a cost-effective solution for generating multiple output bias supplies below 15 W. Since the Fly-buck uses a synchronous buck converter with coupled inductor windings to create an isolated output, the transformer is smaller and better matched in terms of primary and secondary turns ratio. Additionally, the Fly-buck eliminates optocouplers or auxiliary windings because the secondary output closely tracks the primary output voltage.
Basically, as shown in Figure 1, a fly-buck topology transformer X1 is created by replacing the output filter inductor in a synchronous buck converter with a coupled inductor or a flyback. The voltage on the secondary winding of this transformer is rectified using diode D1 and capacitor COUT2. As discussed in Reference 1, the Fly-buck topology can be extended to generate multiple isolated auxiliary outputs.
Figure 1: Typical fly-buck converter topology.
Essentially, the primary output voltage VOUT1 of the fly-buck is similar to a buck converter as follows:
Similarly, the secondary output voltage VOUT2 is as follows:
where VF is the forward-secondary rectifier diode voltage drop, and N1 and N2 are the number of turns of the primary and secondary windings of transformer X1, respectively. As shown in equation (2), the secondary output (VOUT2) closely tracks the primary output voltage (VOUT1) without the need for additional transformer windings or optocouplers for feedback across the isolation boundary.
Also, as described in Reference 1, TON is the time when the high-side switch Q1 is on and the low-side switch Q2 is off. Similarly, TOFF is the time when the low-side switch Q2 is on and Q1 is off. During TON, the current in the secondary winding is zero because the secondary diode is reverse biased by a voltage equal to VIN×N2/N1. The current in the primary winding is the same as the magnetizing current, similar to the buck converter inductor. The current calculations in both windings are detailed in Ref. 1.
The topology shown in Figure 1 can now be easily implemented using traditional synchronous buck regulators such as TI’s LM5017, which include high-side and low-side MOSFETs in compact packages such as the WSON-8 and PowerPAD-8. Since the LM5017 uses a constant on-time control scheme, no loop compensation is required, it provides excellent transient response, and can handle high step-down ratios. This buck regulator is rated for an input range of 7.5 V to 100 V.
A typical isolated Fly-buck DC/DC converter using the LM5017 is shown in Figure 2. It is a dual output circuit with 10V as primary output voltage VOUT1 and secondary output voltage VOUT2. In this design, the primary load current is 100 mA and the secondary load current is 200 mA. The switching frequency is 750 kHz.
Figure 2: A typical isolated Fly-buck DC/DC converter based on TI’s 100 V synchronous buck regulator LM5017.
Component calculations for this dual-output isolated Fly-buck DC/DC converter are given in Reference 1. Based on the above specification, this reference voltage provides component values for an LM5017 based Fly-buck converter. These values are implemented in the schematic of an isolated Fly-buck DC/DC converter based on the LM5017, as shown in Figure 3. This reference also provides measured experimental efficiency results for the Fly-buck circuit in Figure 3, shown in Figure 4.
Figure 3: Complete schematic with component values for an isolated Fly-buck DC/DC converter based on the LM5017.
Figure 4: Experimental efficiency performance of an LM5017-based isolated Fly-buck with 10 V output and 750 kHz switching frequency.
It is observed that the efficiency of the Fly-buck converter is higher when the input voltage is lower, which corresponds to a smaller step-down ratio. As the input voltage increases, the efficiency starts to drop significantly because the step-down ratio increases, which results in higher conversion losses. In contrast, the change in efficiency from low load to full load increases as the input voltage increases. Therefore, when the input voltage is as high as 72 V, the efficiency drop at low load is more pronounced.
For applications requiring slightly lower output voltage and higher output current capability, TI offers the LM5160A, a synchronous buck converter with an input voltage range of 4.5 to 65 V and a maximum load current of 1.5 A. The LM5160A integrates high-side and low-side MOSFETs , built-in constant on-time control scheme without loop compensation, supports high step-down ratio and fast transient response.
In short, for low power, multi-output isolated buck DC/DC converter solutions from various input voltages, when very tight regulation, a fly-down buck converter is the better choice. Not critical for regulated output voltage, but simplicity, cost and board space are critical for the application.