IC 30V 400V bipolar voltage regulators Electric vehicles, large energy storage battery packs, home automation, industrial and telecom power supplies all need to convert high voltages to ±12 V for amplifiers, sensors, data converters and industrial process control bipolar power rail requirements for the controller. One of the challenges in all of these systems is to build a compact, high-efficiency bipolar regulator that operates from -40°C to + 125 °C, which is especially important in automotive and other high ambient temperature applications .

Linear regulators are well known and often top the list of bipolar power supply alternatives, but they are not suitable for the high input voltage, low output voltage applications described above, which are primarily driven by linear regulators at high step-down ratios. caused by heat dissipation. Additionally, a bipolar solution requires at least two integrated circuits (ICs): a positive-output linear regulator and a negative-output converter. A better solution is to use a single switching regulator that produces two outputs from the higher input, with good efficiency and regulation, while still fitting into tight spaces and reducing cost.

This article presents two compact circuits that use a single high-voltage LT8315 converter to generate ±12V outputs from a wide input voltage range of 30V to 400V. One circuit is an isolated flyback topology and the other is a non-isolated buck topology. The LT8315 itself is a high voltage monolithic converter with integrated 630 V/300 mA MOSFET, control circuit and high voltage start-up circuit in a thermally enhanced 20-pin TSSOP package.

Isolated Bipolar Flyback Regulator Without Optocoupler

Flyback converters are widely used in multiple output applications to provide electrical isolation, improve safety, and enhance noise immunity. The output can be positive or negative, depending on which side of the output is grounded. Traditionally, output voltage regulation has been achieved using optocouplers to transfer information from the secondary side reference circuit to the primary side. The problem is that optocouplers greatly increase complexity and reduce reliability due to propagation delay, aging, and gain variation. Typically, the output connected to the feedback pin of the IC dominates the regulation loop, while other outputs are loosely controlled through the transformer windings, resulting in poor regulation of these outputs.

The LT8315 eliminates the need for an optocoupler and samples the isolated output voltage from the flyback from the third winding of the power transformer. In addition, it can detect the output voltage when the secondary side current is almost zero for excellent load regulation. In dual output designs, this special detection scheme allows tight regulation of each output (both outputs can dominate regulation). Therefore, typical ±5% load regulation is very easy to achieve.

The LT8315 solution shown in Figure 1 operates in quasi-resonant boundary conduction mode. The primary side MOSFET has very low conduction losses because the MOSFET turns on when the switch node ringing reaches its valley value. There is no diode reverse recovery loss on the secondary side. The 3 kV reinforced insulation transformer is the only component on the entire isolation barrier, which increases system reliability and meets stringent high-voltage power isolation requirements. Figure 2 shows the full-load efficiency curves at different input voltages. The flyback converter achieves a peak efficiency of 85.3% when the input is 70 V and both load currents are 50 mA.

Figure 1 shows the complete schematic of a flyback converter with a wide input range of 30 V to 400 V. It outputs ±12 V and maintains very accurate control at load currents from 5 mA to 50 mA. The peak efficiency of this flyback converter is 85.3%, as shown in Figure 2.

Figure 1. A complete ±12 V/50 mA isolated flyback converter for a wide 30 V to 400 V input range.

Figure 2. Full-load efficiency versus input voltage for the flyback converter shown in Figure 1.

Figure 3. Schematic of a non-isolated dual-inductor buck converter using a single LT8315 IC: from 30 V to 400 V input to ±12 V output (30 mA each).

Generate isolated or non-isolated ±12 V outputs from 30 V to 400 V inputs with a single IC

Non-Isolated Bipolar Buck Regulator Using Dual Inductors

The LT8315's high voltage input capability enables non-isolated solutions with off-the-shelf inductors. Figure 3 shows a dual-inductor buck regulator that requires only a few components. The converter accepts a wide input voltage range (-30 V to 400 V) and produces an output of ±12 V/30 mA. When the input is 30 V, both outputs of the circuit can achieve up to 87% efficiency at full load.

In this topology, the LT8315's GND pad is intentionally left ungrounded, but connected as a common switch node that drives both outputs. During PCB layout, the LT8315's GND pad size should be limited to the exposed pad area to reduce EMI to other components, since the GND trace is the relatively noisy switching node in this topology. Diode D2 forms a feedback path with two 1% resistors on the FB pin to regulate the positive output voltage. D2 is necessary to prevent the FB pin from discharging whenever the MOSFET is turned on. The resistive divider does not need to consider the forward voltage drop of D2 because the forward voltages of D2 and D3 are equal and can cancel each other; therefore, the feedback network tracks and tightly regulates the positive output voltage.

The negative rail includes a low voltage coupling capacitor CFLY, a second inductor L2, a circulating diode D4 and a negative output capacitor CO2. According to the inductive volt-second balance of the CO1-L1-CFLY-L2 circuit loop, the average voltage across L1 and L2 is zero, so the voltage across the coupling capacitor CFLY is equal to the positive output voltage. CFLY charges L2 during MOSFET on-time, while D4 provides a discharge path for L2 during MOSFET off-time. The negative output voltage is regulated indirectly based on the CFLY voltage (which remains constant and equal to the positive output voltage). As shown in the regulation curve in Figure 4, when the positive output voltage is loaded with a full load of 30 mA, the negative output voltage maintains ±5% regulation over the load range from 3 mA to 30 mA for different input voltages.

Figure 4. Negative 12V load regulation curves for the dual-inductor buck converter shown in Figure 3 at various input voltages.

in conclusion

This article describes two bipolar converter solutions for a wide input range of 30 V to 400 V: one is isolated and the other is non-isolated. The LT8315 is suitable for both schemes because it has a built-in high-voltage integrated MOSFET and high-voltage start-up circuit, eliminating the need for an optocoupler feedback loop. Other features of the device include low-ripple Burst Mode™ operation, soft-start, programmable current limit, under-voltage lockout, temperature compensation and low quiescent current. The high level of integration of the LT8315 simplifies the design of high voltage input and bipolar output circuits in a variety of applications.