Power Suppliesvoltage converter

Small Power Converter Schematic Circuit Diagram

Converting Existing Positive Supply Voltage with DIY Transformer

This converter offers the flexibility to elevate, lower, or transform an existing positive supply voltage into a negative potential. The newly generated voltage is electrically isolated from its source using a custom-made transformer wound around a G2-3FT12 toroid. The primary winding, comprising 30 turns, determines the number of secondary turns, denoted as ‘n,’ using the formula n=30Uo/U1, where U0 represents the desired voltage, and U1 represents the input voltage. To compensate for losses, add 10-20 turns to the calculated number. To adjust the output voltage, a P1 component can be utilized.

Both windings can be wound using 0.3 mm diameter enameled copper wire, ensuring an even distribution of turns along the core. The CMOS Schmitt trigger NAND gate drives the transformer, which is modified into a rectangular-wave generator by R1 and C1. MOSFET T1 functions as the output stage. R2 and P1 provide additional charging current for C1, controlling the duty factor of the rectangular signal. It’s crucial to maintain a signal frequency of approximately 220 kHz, with a duty factor lower than 0.5.

Energy Transfer Mechanism in the Converter Circuit

When T1 is activated, energy is transferred to both the secondary winding and stored within the magnetic field. When T1 ceases conduction, the energy stored in the magnetic field is then transferred to the secondary winding.

 Small power converter Schematic diagram

Optimizing Duty Factor for Efficient Energy Transfer

To ensure efficient energy transfer and prevent the risk of core saturation. It is imperative to maintain a small duty factor. This precautionary measure guarantees that all energy stored in the field is transferred before T1 switches on again. Failure to do so could lead to an escalating residual magnetic field, causing core saturation and reducing overall efficiency. Additionally, inadequate inductance at the primary side results in a significant rise in current through T1, posing a threat to the transistor’s functionality.

Managing Current Flow Through T1 and Secondary Load

Managing the current flow through T1 is crucial, especially under heavy secondary loads. It is essential to keep the average current through the primary below 150 mA, even though the peak current may be several times larger. Calculating the maximum secondary load current is straightforward based on the turns ratio. In the provided configuration (including 10% extra turns at the secondary side), the secondary load should not be less than 80 Q to prevent dangerous rises in current through T1.

Avoiding Heavy and No Load Situations

Preventing both heavy and no load scenarios is essential for the converter’s stability. Excessive secondary load strains the system, leading to dangerous rises in current through T1. Conversely, no load situations force the energy stored in the magnetic field to move to C2, where it is stored in the electric field. This accumulation can elevate the charge on C2 and the voltage across it, potentially disrupting the connected circuit. The specified value of 1.5 serves as a proportional guideline, much like the maximum load current, based on the turns ratio.

Choosing Suitable Rectifier Diodes

To handle the 220 kHz frequency effectively, it’s important to use Type 1N4148 rectifier diodes, as they have the necessary speed (unlike 1N400x types). These diodes can withstand a constant current of 200 mA, with a peak tolerance of 400 mA.

Efficiency and Voltage Considerations

The converter’s efficiency, operating with a 15 V supply voltage, is approximately 65%. However, this efficiency decreases to around 50% when the load current is low and also diminishes with a supply voltage lower than the specified value.

Limiting Input Voltage and Managing Current

Ensure the input voltage does not exceed 15 V, preventing IC1 and T1 from surpassing this limit. For an 80 Q load, the current drawn from a 15 V source is roughly 165 mA. During testing, the prototype performed well with P1 turned fully to +, suggesting the possibility of slightly reducing the value of R2.

Adjusting Rectangular Voltage Duty Factor

If necessary, the duty factor of the rectangular voltage can be adjusted using P1. However, it’s crucial to monitor the current through T1, preferably using an oscilloscope. If the current rises too rapidly, indicating core saturation, P1 should be adjusted slightly backward. Keep in mind that setting P1 to a critical position means even a slight change in loads can push the core into saturation, leading to significant consequences.

 

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