Help reverse engineering strange induction transfer power+program circuit - Page 1
Forum section: electronics / projects

author: Houseman
author_rank: Regular Contributor
timestamp: July 31, 2017, 11:45:33 am
content: Dear forum. I really need some help from you finishing understand this circuit I briefly explain:It’s a Chinese Small Desktop FAN powered via USB that displays text on the fan LED while rotating due to Persistence of Vision effect.User can program the Text via PC program and then upload it via USB cable.The fan is quite cheap but under the hood I think there are some remarkable smart engineering solutions; in order to prevent counterfeit attempts the producer has wiped off with Nitrogen Acid EVERY signature on EVERY IC of the two boards (!!!) I imagine in the factory production line there must be one operator that every day has the specific task to wipe off all the marks…There is aSTATOR PCB that:- Powers the fan,- Has the USB translating chip,- And provides power to theROTOR PCB:This consists of power circuitry, a MCU, an EEPROM i2c memory and 11 SMD RGB LED.During normal use (not programming mode) the MCU reads the pattern from the EEPROM and Lights up the LED according to the code; so the real brain is placed on the moving rotor head.In order to prevent copper brushes wearing, expensive slip rings, the power and the programming code is transferred from the stator to the rotor head via two inductor coils (placed one into another).I have decoded almost part of the circuit but I need some help in order to understand the transferring power and signal logic: (photo1) There is a small unknown IC 8pin SOIC chip that drives two MOSFET ( these two driving the LC circuit) one P-mos over a N-mos in a .. Push-pull? Semi totem-pole? (please correct me) configuration.This chip provides when pin 3 is pulled high a “dual” (I mean complementary) 66kHz square waves on pin 6 and 7 each delayed one by few nanoseconds (photo2).No extra external capacitor is used for the task. Just a voltage divider by two resistors between pin 3 and vdd.On the “receiving part” of the Rotor head there is a full bridge 4-diode that rectifies the L sinusoidal 14,2Vpp waveform into 5V DC providing power to the moving board. On one half of the bridge a resistor divider taps the signal that contains the receiving “programming path” into the MCU (photo3)The thing I need is understand is the 14 pin SSOT USB to some sort of “SERIAL” conversion” for transferring data to the rotor MCU via induction.During programming phase the pin 3 of IC (that is normally held HIGH) is “MODULATED” by the BYTES that are transferred from the USB chip to the MCU in a 1-wire mode manner. Photo shows the 1-ch is the pin3 tap , 2-ch NMOS tap 3-ch PMOS tap (photo3).I suppose the chip is only a uncommon USB to Serial converter that transfers the programming byte using only the TX channel. No flow control is used.I have analyzed via software the BYTE transfer but apart from some frame error I am stuck on the logic that stays behind.NOW THE QUESTIONS:A. What kind of modulating technology is this? Modulating the Byte and at the same time providing power to the board?B. What strange (at least for me) 8pin IC MOS driver provides such two complementary square waves? Can You give me some examples? I know maybe it is a quite common method that is unknown for me.Any help would be very appreciated.Thank You as always.Steve

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2017, 02:17:09 am
content: Could just be an MCU.VDD is 5V?Looks not very strange to my eyes.Tim

author: Houseman
author_rank: Regular Contributor
timestamp: August 03, 2017, 12:12:41 pm
content: T3sl4co1l YOU ARE THE MAN!! Thank you a lot!!!YES OF COURSE!!! COULD BE.I wassearching the CHIP database and maybe it's so simple!!Maybe even a ST PIC 8pin MCU (or a chi-clone)VDD is 5V and the PIN displacement could be right. VDD pin 1 GND pin 8.The other the GPIO pin.Will try it tonight with my programmer!!Thanks you alot again!!You opened my eyes!!!What kind of driving such mosfet to load LC circuit?!? Simil PUSH PULL technique?Have you ever seen such a configuration?Ciao

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2017, 08:24:36 pm
content: Called "half bridge", since it's one leg of an "H bridge". In this case, a complementary (PMOS and NMOS) one, because the supply voltage is the same as the logic level.Neat to see they still drive it with dead time (time delay between the low side turning off and the high side turning on, etc.).If both gates are driven together (they rise and fall at the same instant), they will both be on, during the moment that gate voltage is beyond Vgs(th) (typically 0.8-2V for small MOSFETs like would be used here), drawing a large gulp of supply current. Or, often, a small enough time is spent shorting out the supply that it's not a problem, and combined drive is okay.The complementary half bridge, with gates wired together, is also simply an unbuffered CMOS logic inverter.Tim