A wide-range, linearly-programmable DC/DC converter

This Design Idea presents a simple, low-cost, highly flexible, and scalable DC/DC converter.

The output is linearly programmed by an external, ground-referenced voltage.
The design is a two-phase flyback converter powered by an input of 15 V to 30 V.
It provides an output voltage ranging from 0 V to 200 V with a maximum power output of up to 60 W.
The system is built around a MIC3808/MIC3809 or a similar push-pull PWM controller.

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The Challenge

Some applications demand significant DC power delivered at a precisely controlled voltage that may vary over a wide range. Traditional solutions for this requirement often involve:

Linear regulators: Suffer from low efficiency and high power dissipation.
PWM step-down converters: Can only generate voltages lower than the rail voltage and lack a voltage feedback loop unless explicitly implemented.
Digital potentiometers / multiplying DACs: An alternative is substituting the resistive feedback dividers of power control ICs with digital potentiometers. However, this requires digital rather than analog control. It also causes massive changes in the voltage feedback loop gain over the adjustment range, which can destabilize the output voltage.

The Proposed Solution

This simple, generic solution converts power control ICs with a voltage feedback input to allow for wide-range, linearly adjustable DC/DC conversion.

In the preferred implementation, a small-value, voltage-controlled current source is used to remove current from the sensing point of the DC/DC controlling loop’s voltage feedback divider.

This proposed current source is built using a small MOSFET (such as a 2N7002) and an operational amplifier.
It is often preferable to use op-amps with a voltage input that extends to the lower rail, which is ground in this case (e.g., LM358).

Design Considerations

Constant transfer ratio: Due to the high differential impedance of the current source, the transfer ratio of the voltage feedback divider remains constant.
Linearity and stability: When considering the stability of the feedback system, one must pay attention to the fact that the DC/DC converter is not fully linear. Typically, sufficient output capacitance must be provided to ensure feedback stability and low ripple across the entire voltage and current range.
Feedback depth: Because a high-impedance current source is used for injection into the resistive divider, the effective depth of the negative feedback loop varies less than it would with a programmable resistive divider.
Discharging capacitance: Some circuits may struggle to discharge output capacitance when the DC/DC converter is suddenly programmed to a lower voltage under a weak load. These cases may benefit from the use of bidirectional converters utilizing synchronous rectification.

Concept

As shown in the conceptual schematic (Figure 1), the target output voltage V_out as a function of current I is given by the following formula:

V_out(I) = V_ref * (1 + R1/R2) + R1 * I

Figure 1 This diagram shows a conceptual schematic of the proposed design.

Implementation

As detailed in the full schematic (Figure 2), the output voltage control remains linear up to the clipping condition, which is typically the preferred behavior for linear control.

Figure 2 The full schematic details of a DC/DC converter providing up to 60 W in a voltage range of 2 V to 200 V, linearly controlled by a ground-referenced voltage input.

For this specific implementation, the output voltage is calculated as:

V_out(I) = V_ref * (1 + R7/R8) + R7 * I – (V_d – V_ref) * R7/R1

The current I, dependent on the input voltage V_in, is:

I = I(V_in) = min(V_ref / R10, (V_in / R10) * (1 + R9/R11))

During a V_in overdrive situation, the maximum output voltage for this circuit corresponds to the MOSFET (Q3) being fully open. The output voltage is then:

V_out = V_ref * (1 + R7/Rp)

where Rp is calculated as:

Rp = (R8 * R10) / (R8 + R10)

Advantages of the two-phase flyback approach

This DC/DC converter is built as a two-phase flyback using a low-cost Micrel MIC3808 controller (Figure 3). Similar controllers from other manufacturers, such as the Texas Instruments UCC38083, can also be used in similar configurations.

Figure 3 A PCB implementation of the design (left) was fabricated and in-lab tested (right).

Using a two-phase flyback approach offers several advantages over one-phase flyback or boost converters:

Magnetic core saturation: An extensive, guaranteed 50% off-time for each transistor allows for better handling of loading conditions across a wide range of output voltages, preventing magnetic saturation of the core.
Filtering: A two-phase converter provides better conditions for input and output filters, which is especially important under highly variable load situations.
Component sizing: Using two phases allows the delivery of higher power using low-cost, mass-produced SMD coupled inductors rather than larger, more expensive magnetics.
Cost efficiency: The cost of the DC/DC controller IC is often lower for push-pull type controllers compared to one-phase flyback or boost controllers. The cost of adding the second phase (MOSFET, inductor, diode) is usually much smaller than the savings on the controller.

Experimental data

Here are some experimental measurements results from this board (Figures 4-6).

One can see perfect linear control of the output voltage.
The idle power consumption is about 0.36 W while high power efficiency is approaching 78%.

Figure 4 This graph illustrates output voltage vs input voltage for the design.

Figure 5 This graph illustrates output power vs input power for the design.

Figure 6 This table details actual measurements for the lab-tested PCB prototype.

Qualifier

One thing that should be mentioned is that at light power with a small output capacitor, the DC/DC output may go through some slow (10-20 msec) and irregular oscillations. An example 90 V, 1.5 W output is shown in Figure 7.

Figure 7 Example oscillations at light output power (1.5 W) with a small output capacitor.

The ripple is quite small—0.1 V or so—and disappears in higher power output applications. Unfortunately, the non-linear nature of energy conversion in flyback DC/DC configurations makes them impossible to avoid across some subset range of all possible outputs.

To avoid this issue, the general recommendation is to have a large enough output capacitor; what’s important here are the output load characteristics. One can also change the feedback loop gain by setting a suitable value for the compensation capacitor (C8 in the schematic), but this alternative approach will also slow down the reaction time of the DC/DC feedback.

Nikolay Pavlov is an electronics/sensors hardware R&D engineer specializing in circuit analysis, component selection, electromagnetic modeling and signal processing. Outside of professional hardware design, he is an amateur radio operator with a deep interest in RF technology, Morse code, and shortwave radio operations.

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