Coupled inductor expands the circuit diagram of the DC/DC converter

Photocoupler

Overview:
Smaller and more efficient SEPIC
Although there is always a need for converters that can regulate the output voltage between high and low input voltages (eg, unregulated 12V power supplies on the wall), and the DC/DC single-ended primary inductor converter (SEPIC) topology is not What's new, but it's really only recently that it became popular. Although we can configure any boost converter/controller as a SEPIC, it has only recently gained widespread use. Two factors have contributed to the popularity of SEPIC: (1) IC manufacturers have begun to manufacture more boost controllers with current mode control to simplify compensation; (2) inductor manufacturers have begun to manufacture many that can be minimized. Single-package coupled inductor for the total PCB volume of the converter. In particular, after switching to a coupled inductor, many power supplies with two separate inductor applications can be reduced by a third. Figure 1 shows a SEPIC using TI TPS61170 and Wuerth 744877220.

Figure 1 SEPIC using TI TPS61170 and Wuerth 744877220.
Even more compelling, a SEPIC with a 1:1 coupled inductor forces the inductor ripple current to separate between the two windings, allowing two separate inductors to require 2/1 of the inductor, producing the same ripple current. Coupled inductors have lower DC resistance than two separate inductors with twice the inductance value in the same size package, which helps improve overall converter efficiency. In particular, the SEPIC efficiency shown in Figure 1 exceeds 91% for 15-V input and 12-V, 325-mA output. See Reference 1 for more details.

The smaller ZETA converter uses two inductors and one coupling capacitor. The ZETA converter has the same boost buck as SEPIC, but uses a buck controller instead of a boost controller. Figure 2 shows the TI TPS40200 and Coiltronics DRQ74 used in the ZETA structure. Like the SEPIC, thanks to the separate inductor ripple current, the same ripple current requires only half the inductance of the ZETA converter. As with SEPIC, its overall power supply is less than one-third smaller than using two separate inductors. Since the output inductor current continuously flows into the output of the ZETA converter, the output of the ZETA converter has a lower ripple than the SEPIC of the same inductor. Therefore, ZETA may be more suitable for low noise applications than SEPIC. See Reference 2. for more details.

Figure 2 ZETA converter using TI TPS40200 and Coiltronics DRQ74

Separating rail power supplies that match positive and negative rails are common requirements for many industrial applications, especially for amplifiers. We can configure a wide input range buck converter to provide a negative output voltage. By replacing the inductor of this inverting buck converter with a coupled inductor and adding a diode and capacitor, this inverting buck converter can be turned into a dual output converter. Figure 3 shows the TI TPS54160 and Coilcraft 150-μH MSD1260 used in this way. As long as the load on each rail is slightly close, we adjust the difference between each rail instead of adjusting each rail individually, but Coupled inductors can help provide excellent regulation for each rail. For more details, please see Reference 3.

Figure 3 uses a separate rail buck converter for the TI TPS54160 and Coilcraft MSD1260.
The output voltage of the higher output voltage integrated FET's DC/DC converter is limited by the converter's switching current rating. Connecting a 1:1 or higher turns-coupled inductor to the converter's switch (SW) pin extends the effective output voltage range of all boost converters. For example, Figure 4 shows a TI TPS61040 boost converter with a 30-V absolute maximum current rating that provides 35V or higher voltage and a 1:2 coupled inductor Coilcraft LPR4012-103B. When the multiple winding ends of the structure are connected in series with the diode, the single winding inductance - and the resulting converter switching FET - voltage is only one-third of the output voltage, ie the negative input voltage.

Figure 4 TI TPS61040 and Coilcraft LPR4012-103B with a larger output voltage range

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