Switching power supplies, low-power self-oscillating is simple to implement and very economical. Indeed, these are self-power oscillator and a regulator / controller specific such as the UC3843, the L6562A, the TL494 or SG3525.
Principle of self-oscillating power hysteresis
If you have an electric heating thermostat in your home, it's easy to understand the principle. If you do not because you warm otherwise, it is also very simple to understand the principle. Imagine you want a temperature of 20 ° C. Hysteresis of 1 ° C for example is on the thermostat.
The room temperature, initially cold, is measured by the thermostat. The heater is then turned on and stays on until 20.5 ° C. He cut and room temperature gradually decreases, according to its thermal inertia and leakage (open windows, etc.). When the temperature reached 19.5 ° C, heating off again until it reaches again 20.5 ° C. It cuts across time, and so on. The alternating switching-off of heating is intermittent operation whose frequency depends on external factors.
Diagram of SMPS Buck 3.3V to op amp
Here's the pattern as expected:
Principle of switching power supply
- Power supply stabilized LMV331
The controller S812C30AMC Seiko was chosen for its very low vacuum (2UA) and its small size (SOT23). It supports 16V input: this limits the voltage power input. C1 and C2 stabilize the operation of the controller. The controller supplies the LMV331 op amp comparator and also serves as a reference voltage (+ input connected to the LMV331 bridge R1/R2). There are about 1.65V + input of the LMV331.
SMD SOT23
Any other controller can be chosen as it provides between 3V and 5V.
- Comparator Hysteresis
The op amp operates as a comparator here, not in linear, despite some misleading appearances. Indeed, to maximize the switching frequency, hysteresis has been made zero: no resistance comes from the collector of Q2 to the input of + LMV331.
Suppose the supply voltage is too low. The potential of the entrance - is lower than the + input. The output of the LMV331 is high (open collector), then Q1 is conductive and allows the conduction of Q2. R5 accelerates the blocking of Q2. Its value should not be too different from 3.3kOhms (2 to 5kohm). The output voltage will increase until the potential of the input - the input exceeds that of +. The output of the LMV331 then goes low, making block Q1 and Q2 and. The operation is all or nothing. C4 adds a transient effect to bring more changes in input voltage -: this increases the frequency.
In reality, the switching frequency is due to delays in the switching transistors and op amp and the values of the inductor and output capacitor.
Q1 only serves to support the supply voltage. Indeed, while the LMV331 stand on its output 16V blocked, it could happen in Q1.
Choice of components of the switching power supply
Here, the choice was made for a very small load consumption (0.1mA max.) And a small footprint (SOT23). Inductance is the component that will be the largest (6x6x3mm typical).
The inductor must withstand 150mA without saturating (consider triple the average current output).
Resistors and capacitors have values ultra standard.
Measurements on the switching power supply
- Performance
When empty, consumption is approximately 123uA. To 10mA output, the current consumption on the 12V is 3.61mA. The yield then reaches 76%.
Performance of the switching power supply according to the load (0 to 10mA)
- Switching frequency
If the current output power is low, the output voltage falls slowly and the switching frequency is low, the recharge cycles of the output capacitor are rare.
Output voltage of the switching power supply (principle)
If the output current is higher, the output voltage drop and faster recharging cycles are more frequent.
Output voltage of the switching power supply (principle)
- Peak to peak ripple of the output voltage
Increasing the maximum input voltage of the switching power supply
A transistor ballast can be added to increase the maximum input voltage of the power supply.
Buck power that supports up to 45V input
This time, they are the transistors that limit the input voltage because of their Vce max (45V). The yield will be slightly reduced for two reasons:
- A vacuum because of the use of the bridge (resistance + zener) which supplies the base of transistor ballast (BC846B choose an example).
- A heavy load, the switching losses are also somewhat lower yield (60 to 70%).
Choice of inductance
We can reduce the output inductor 0.1mH up if the return is not a priority. The yield increase from 76% to 60% for 10mA output.
Advantages of this switching power supply 3.3V
- Very low cost (0.65 euro 1.40 euro against industrial production for a regulator similar to Linear Technology)
- Ultra low-load consumption (0.1mA)
- Very decent performance (76% at 10mA output, 12V input)
- Input range up to 45V possible scope
- Output current up to 50mA.
Disadvantage of this switching power supply 3.3V
- No protection against short circuits
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