Beautiful S35U-C humidifier switching power supply circuit diagram and analysis

Probe domestic switch needle KG-300K needle head diameter is 3.0mm normally open switch needle

The circuit board of the beautiful S35U-C humidifier is shown in the figure below.

It can be seen that the 220V AC mains supply is sent to the D1~D4 full-bridge rectification and capacitor C2 filter through the fuse Fl and the thermistor K1, and the 300V DC high voltage is obtained to provide power for the drain of the switching power supply. Kl is a temperature sensitive component with a negative temperature coefficient (it is characterized by large cold resistance and low thermal resistance). Its role is to limit the amount of starting surge current generated in the circuit just after the power is turned on. The switch tube with transformer load prevents instantaneous breakdown due to excessive starting current.

The self-excited multivibrator in the switching power supply operates from 30 kHz to 50 kHz. The high-frequency oscillating voltage is stepped down by the transformer TC, and the voltage across the secondary winding N5 is smoothed by the D6 half-wave rectification and C3 filtering to output the DC voltage. For 12V fan motor: winding N4 and N5 are connected in series and then output DC voltage 36V through D12 rectification and C4 filter. As a power source for the ultrasonic oscillator. The capacitor C6 connected in parallel with D12 is to eliminate high frequency ripple.

The switching power supply consists of a self-excited flyback field effect transistor Q3, a high frequency transformer TC winding N1 and a secondary winding N2, and related components such as resistors, capacitors, diodes, and voltage regulators. Its working principle is:

300V DC high voltage is applied to the drain D of 03 via the N1 winding, at the same time. +300V through resistors Rl, R2, R7. After the voltage dividers DWI and DW2 are divided, the required forward bias voltage is supplied to the control gate G of 03, and then 03 starts to conduct. Thus, in the high-frequency transformer N1, there is a drain-source current ID flowing from the 300V DC power supply through the FET 03 through the source resistor R1" and R8, R6.Q3 I. The voltage is induced through the N1 winding of the transformer. U21, through the coupling of the transformer, the voltage shown in the figure below is induced on the N2 winding. This voltage will provide a positive bias to the control gate G of Q3 via resistor R4, diode D7, R7. Increasing the drain current ID of Q3 causes the induced voltage on the primary winding of the transformer to become larger. This cycle reciprocates to form a strong positive feedback process.

The induced voltage UL=Ldi/dt formed on the transformer winding. When the current ID flowing through the transformer winding N1 linearly increases, the induced voltage U. Will remain unchanged and equal to the input voltage. But because the above positive feedback process is very strong. So 03 will quickly enter the saturation conduction state. This means that the growth rate di/dt of the drain current ID will be reduced. As a result, the induced voltage on the primary winding of the transformer is reduced, causing the gate of the FET Q3 to drop and the drain current lD of 03 to decrease. From then on, the drain current is l. The rate of change will change from a positive value to a negative value. As a result, the induced voltage generated on the N34 winding is reversed. The reverse polarity voltage U34 passes through R4, c5, and R7. Make the gate voltage of FET Q3 quickly lower than the turn-on voltage. Q3 quickly entered the cutoff state. This completes an oscillation cycle. This cycle constitutes a self-excited multivibrator.

The high-frequency transformer TC of the humidifier DC stabilized power supply works in the state of energy storage of the inductor. When the FET Q3 is turned on, there is a current 1D flowing in the TC winding N1, and at the same time, with the windings N3, N4, N5 The connected diodes are all in a reverse bias state, which is equivalent to an open circuit without current flowing. In this case, the primary winding can be regarded as a pure inductance. Therefore, the energy provided by the 300V DC high voltage during Q3 conduction is stored in the winding N1 when Q3 is in the off state. The voltage induced in all secondary windings of the high frequency transformer is reverse polarity. At this time, the rectifier diodes in the secondary winding circuit are in a forward bias state, and each of the rectifier filter circuits transmits energy to the load.

The negative feedback control circuit consists of a photocoupler ICI (817C), a precision regulator IC2 (TIA31), a bipolar transistor Q1 (A1015), Q2 (C1015), and related components. The equivalent circuit of the key device TIA31 is shown in the side diagram fH). When the KA terminal of the TL431 is applied with a positive voltage, the current flowing through the TLA3l will increase as the voltage applied to its RA increases.

The induced voltage of the secondary winding N4+N5 of the high-frequency transformer is rectified and filtered. The output DC voltage is fed a 36V voltage to the optocoupler via resistor R13. The voltage forms a current path through the optocoupler LED and its parallel resistors R12 and TL431. At the same time, the output DC voltage is divided by R14 and R11, and then sent to the control terminal R of the TLA 31 as a sampling voltage. If, for some reason, the DC voltage of the rectified output increases, the sampling potential sent to the control terminal R of the TIA3I must rise. The increase in the potential at the R terminal causes the KA current flowing through the TLA 31 to increase. Obviously, the increase in current flowing through TLA3l will inevitably increase the brightness of the LEDs in the optocoupler. Thus the equivalent resistance of the phototransistor in the optocoupler will be reduced. The collector voltage of the phototransistor is from the DC voltage of the off-frequency transformer winding N2 filtered by the rectifying diodes D13, R9 and C10: its emitter is connected to the base of the transistor Q2, because the equivalent resistance between the phototransistors ce is reduced. When Ib2 ↑ → lc2 ↑ → Ib1 ↑ → Ic1 Q of Q2, such a change of the transistors Q1, Q2 causes the gate potential of the pulse width modulation switch transistor 03 to decrease. In severe cases, transistor 02 will enter a saturated conduction state. This forces Q3's turn-on time to shorten and advances into the cut-off phase. Thus, the output voltage pulse width induced in the secondary winding of the high frequency transformer will be narrowed. From the pulse width regulation working principle, when the driving pulse width is narrowed, the D12 output voltage will fall back to its normal value. Conversely, when the D12 voltage output drops for some reason, the pulse width widened drive voltage pulse is rectified by diode D12 and filtered by capacitors C6 and C3 to cause the output voltage to rise again to its normal value. It is because of this negative feedback control of the pulse width modulation control circuit that the DC voltage value of the switching power supply output can be kept stable.

The overcurrent protection circuit of the switch tube 03 is composed of a current limiting resistor R1, resistors R8 and R6, and capacitors CII and Q2, Q1 and the like. For some reason, the drain-source current I flowing through the switching transistor 03 increases and exceeds a prescribed value. One-way pulsation generated across the current limiting resistor R1'. The voltage drop is divided by resistors R6 and R8 and fed to filter capacitor CI1 to form a DC voltage drop. This voltage is applied to the base of transistor Q2. Turning on 02 causes the gate voltage of Q3 to fall below the turn-on voltage, and quickly turns the switch transistor 03 into the off state in advance, shortening the on-time, thereby achieving the purpose of overcurrent protection.

The high frequency transformer N3 winding is a return winding, which has a function of providing a magnetic flux reset for the high frequency transformer during the off period of the switch tube 03, and a function of eliminating the peak voltage on the drain of the switch tube in series with the diode D5.

Usually, the number of turns of N3 is equal to Nl and the polarity is opposite. When Q3 is turned off, the drain voltage rises so that "D5 is turned on. The drain voltage of Q3 is clamped, and the leakage sense energy is returned to the grid. For good results, N3 and N11 must be tightly coupled, clamped. The diode can be turned on quickly, and a high speed switching diode is required.

Midea S35U-C humidifier switching power supply circuit