TL494 magic chip

Description: The circuit operates using a simple passive MOSFET drive. However, increasing the switching frequency or utilizing more powerful MOSFETs introduces challenges due to the absence of load in the gate circuit, which results in gate capacitance. To achieve fast switching, it is essential to charge and discharge the gate quickly. Passive driving has its limitations, necessitating the use of an active charge-discharge circuit. This circuit employs an inverter transistor for proper signaling.

A few comments regarding the schematic: R5 serves as the output load for the TL494, typically around 1K ohms. R1 acts as the load for Q1 and is not critical, also around 1K ohms. Q1 inverts the signal from the controller, while R2 prevents overloading of the totem pole bases. Q2 and Q3 form a complementary pair. R3 is beneficial when multiple MOSFETs are connected to the output, requiring only a few ohms. The transistors used are C945 for Q1 and A733 for Q3, both sourced from older PC power supplies. R4 simulates a load. Connecting a pair of these totem poles to a transformer, as described in an earlier post, can yield a slight performance increase, but care must be taken not to overload the circuit.

There are significant parasitic oscillations present in this circuit, which can lead to malfunction. For instance, R1 can inadvertently activate Q3, causing the MOSFET to remain fully on, resulting in excessive heat. To resolve this, R1, R5, Q1, and R2 should be removed, and a direct connection to pins 9 and 10 should be established. A resistor of 3.9K ohms should be connected to ground from these pins. This modification has proven effective through extensive testing.

The circuit consists of two stages: the first stage is a signal inverter using Q1, while the second stage functions as a totem pole driver. It is important to note that the transistors used are different types: NPN and PNP, forming a complementary pair where one transistor is active while the other is inactive. R2 limits the base current; if this current is too high, it may damage the transistors.

If C1 and C2 refer to the outputs of the TL494, it is advisable to verify that the load resistor R5 is correctly valued (1 to 2K ohms for 15V). Additionally, incorporating base current limiting resistors (not shown in the schematic) is recommended; these should be in the range of a few hundred ohms to a few kilo-ohms. It is also prudent to check Q1 for possible failure due to high emitter-base voltage. In previous experiments, a supply voltage different from 15V was utilized, which may have exceeded the maximum allowable Ueb for many transistors. To mitigate this, voltage should be limited or resistors should be employed to divide the output voltage.

This circuit represents a functional design for a workbench power supply, although it has not been translated into English.Everything is working with simple passive mosfet drive. But if we increase switching frequency or place powerful mosfet (or even several mosfets) There is no load in gate circuit, but there is gate capacitance. And if we need fast switching we must charge and discharge gate very quickly. With passive drive there are limitations. We can use active charge-discharge circuit. Also this circuit uses inverter transistor for proper signaling: Few comments about schematics. R5 is TL494 output load- anything about 1K. R1 is Q1 load, not critical- 1K. Q1 inverts signal from controller. R2 is used to prevent overload of totem pole bases. Q2 and Q3 are complementary pair. R3 are useful if are few mosfets on the output, only few ohms. Q1 and Q3- C945, Q3-A733. All transistors from old PC power supply. R4 load simulation. Connect pair of these totem poles and connect to transformer like in older post. You can notice small increase of the performance. Don`t overload. There are lots of parasitic oscillations in this circuit. These schematic dos not work, why because R1 activate Q3 so mosfet is fully ON hotttttt. Just remove R1, R5, Q1, and R2. Connect directly to pin 9&10 and pin 9, 10 need resistor 3. 9K to ground. Work nice trust me I spend to much time with this. i have built the circuit and have a great pulse on both c1 and c2 but when i connect this driver circuit i have no pulse at the gate of the mosfet. i have checked all connections about 40 times and they are accurate to your schematic here. any ideas michael: c1 and c2 are colector of q1 and q2 The circuilt is quite simple and consist of two stages: first- signal inverter on q1.

Next stage is totem pole driver. Note, that transistors are different npn and pnp. It is so called complementary pair transistors. As structure is different, one transistor is opened while other is closed. R2 limits base current, if it is too much high your transitors will be dead. If you mean by C1 and C2 that it is output of TL494 Check if your load resistor R5 is in proper value (1 2k for 15V). Also, try to add base current limited resistors (not in the circuit image). They act like R3 and are about few hundreds ohms or few k. Check your Q1, it may be dead because of high eb voltage. In my experiments I was not using 15V and many transistors have low max Ueb. Either limit voltage or use resistors to divide output voltage. It is part of quite good workbench PSU, sorry, not translated to English:.