Discrete Design of Isolated 3.3V to 5V Converter

Isolated 3.3V to 5V converters are often used in long-distance data transmission networks, where the bus node controller operates from a 3.3V power supply to save power, while the bus voltage is 5V to ensure signal integrity and provide high drive capability during long-distance transmission. Although isolated DC/DC converter components for 3.3V to 5V conversion are available on the market, integrated 3.3V to 5V converters are still difficult to find. Even if found, these specific converters (especially those with regulated outputs) usually have long product lead times, are relatively expensive, and generally have certain isolation voltage limitations.

The discrete DC/DC converter design shown in Figure 1 uses only a few existing standard components (such as logic ICs and MOSFETs, etc.) to serve as the transformer driver, and an LDO for regulated output voltage. The circuit is prototyped using many through-hole components, making it larger than integrated components, but the board space is greatly reduced due to the use of TI's Little Logic devices. The

main benefits of this design are a smaller bill of materials (BOM) and the freedom to choose an isolation transformer for isolation voltages ranging from 1 to 6kV. Our goal is to provide a low-cost alternative to fully integrated DC/DC converters and stand-alone transformer drivers by making the transformer driver stage a stable output.



Figure 1: Isolated 3.3V to 5V Push-Pull Converter

Operation

Low-cost, isolated DC/DC converters are generally of the push-pull driver type. The operating principle is very simple. A square wave oscillator with a push-pull output stage drives a center-tapped transformer, whose output is rectified and can be used in a stable or unregulated DC form. An important functional requirement is that the square wave must have a 50% duty cycle to ensure symmetrical magnetization of the transformer core. Another requirement is that the product of the magnetizing voltage (E) and the magnetizing time (T), called the ET product in Vμs, must not exceed the typical ET product of the transformer specified by its manufacturer. We must also use a break-before-make circuit installed next to the oscillator to prevent the two transformer core legs of the push-pull output stage from conducting at the same time and causing circuit failure.

Discrete Design

The famous three-inverting gate oscillator consisting of U1a, U2a and U2b was chosen because it is relatively stable with supply fluctuations. Its nominal frequency is set to 330kHz by a 100-pF ceramic capacitor (COSC) and two 10-kΩ resistors (ROSC1 and ROSC2). The oscillator has a duty cycle close to 50% and a maximum frequency fluctuation of less than ±1.5% within the 3.0-V to 3.6-V supply voltage fluctuation range. Figure 2 shows the waveforms at the summing point of ROSC1 and ROSC2 (TP1) and the oscillator output (TP2). All voltages are measured with reference to the circuit reference voltage.



Figure 2: Oscillator waveforms at TP1 and TP2

The Schmitt trigger circuit NAND gates (U1c, U1d) implement the break-before-make function to avoid overlapping of the MOSFET turn-on phase. The other two NAND gates (U2c, U2d) are configured as inverting buffers to generate the correct signal polarity necessary to drive the N-channel MOSFETs (Q1, Q2). Figure 3 shows the complete break-before-make action. To accommodate the limited drive capability of standard logic gates, MOSFETs were selected because of their lower total charge and short response time.



Figure 3: Break-Before-Make Waveforms

The isolation transformer (T1) has a 2:1 secondary-to-primary turns ratio, 0.9 mH primary inductance, and a guaranteed isolation voltage of 3 kV. Figure 4 shows the transformer input and output waveforms.



Figure 4: Transformer Waveforms

Both diodes (D1, D2) are fast Schottky rectifiers that provide low forward voltages while performing full-wave rectification at full load current (VFW < 0.4 V at 200 mA). It is possible to obtain the output voltage directly from the buck capacitor (Cb3) after these diodes. In this case, the output is not regulated, but the efficiency of the DC/DC converter is maximized. However, the designer must ensure that the maximum supply voltage of the affected circuit is not exceeded, which is more likely to occur at low load or open circuit conditions. If the unregulated output voltage under minimum load conditions is too high, a linear regulator must be used after the full-wave rectifier to provide a regulated output supply voltage.

The main benefit of a linear regulator is low ripple output. Other benefits include short-circuit protection and over-temperature shutdown. However, the main disadvantage is that the efficiency is very low.

Figure 5 shows the ripple of the circuit shown in Figure 1 at an output voltage of 4.93 V, while Figure 6 compares the efficiency of the circuit with an integrated DC/DC component with a regulated output.



Figure 5: Output ripple at VOUT = 4.93 V



Figure 6: Efficiency comparison

The following table provides the BOM of a discrete DC-DC converter. Note that the bypass capacitor value is larger than 10 nF, which is commonly used in some low-speed applications. This is because high-speed CMOS technologies (such as AHC, AC, and LVC, etc.) have high dynamic loads, so the bypass capacitor value must be 0.1 μF or higher to ensure normal operation. This is particularly important for the inverting buffer driving the MOSFET, whose bypass capacitor value is 0.68 μF.



Conclusion

A discrete design of an isolated 3.3-V to 5-V DC/DC converter with a regulated output can be a realistic and low-cost alternative to an integrated DC/DC component with a regulated output when there are no board space constraints. The main benefit of a discrete design is the freedom to choose the isolation transformer to meet various isolation voltage requirements.