# High power voltage controlled bidirectional current source circuit

I have tested various amplifiers and motor drivers and none of them are capable of driving my 20Ohm 100mH load at 0-> +/-2Adc (max 0 -> +/-100Vdc) in function of a control voltage (from a 16 bits DAC) with a closed-loop bandwidth of 5kHz. The bias and noise must be <= 1mA. I can give away 10V on the output for sensing.

Honestly I can't see any particular challenge for manufacturers of such equipment to make a product with these sorts of specs and yet I have found nothing suitable - so I am trying to make my own.

I'm looking for a very simple circuit topology that I can design in details - again, I was not able to find anything online after much research other than an application in the LT1990 datasheet that unfortunately can't go beyond 36V. I have two 4-quadrant +/- 100Vdc 2Adc lab supplies with voltage+current limiters so I thought about a simple opamp controlling the gate voltage of a MOSFET push pull based on the readings of a low-side sense resistor, but maybe there is something better?

Space, cost, dissipation are not an issue - I am solely interested in the high-speed performance.

• What is the $\frac{\textrm{d}I}{\textrm{d} t}$ you want to achieve in the load itself (just to confirm what I think I may be reading)? – jonk Sep 9 '17 at 11:37
• @jonk: the slope of the tangent at the origin of a 5kHz sine of amplitude 1A. – Mister Mystère Sep 9 '17 at 12:21
• So, I get $\frac{\textrm{d}I}{\textrm{d}t}=\omega=2\pi f\approx 31500 \frac{\textrm{A}}{\textrm{s}}$. If I'm not mistaken. Have you applied that to your load to see where that points? (Or maybe just being up for just a moment during my sleep period has blinded me?) – jonk Sep 9 '17 at 12:32
• I have done an analysis that yielded a maximum voltage of about 90V when the loop works as expected, if that is what you are asking. Otherwise what do you mean? – Mister Mystère Sep 9 '17 at 13:34
• Does your analysis include and confirm my result above? – jonk Sep 9 '17 at 13:53

It sounds to me like a job for a H bridge made from MOSFETs so that motor current reversal is easily achieved and, with the the main power being current limited with fairly standard circuits. The current limiting could also be done using PWM (and filtered).

• Thanks Andy. I thought of an H bridge as well but seeing as none of the motordrivers can meet these specs (1kHz max with this inductance) and they are all based on 40kHz PWM, I thought I would "go the Bulldozer way" and design something that can be 10 to 50 times faster and more accurate. Don't you think ? – Mister Mystère Sep 9 '17 at 9:53
• The H bridge is just a changeover relay in my minds eye. When the 16 bit number drops to half way it flips over to reverse the motor connections. Below half way and down to zero the motor current increases so you use 15 bits to control current and the 16th bit is regarded as "sign". Clearly you need to convert the 16 bit number to a signed integer and you can do that digitally or in analogue (if you have a DAC output) using a precision rectifier. The 15 bit number then controls current intensity. – Andy aka Sep 9 '17 at 10:17
• I understand what you are saying, but I am not sure this is much more efficient than what is already implemented in the motor drivers I have tested, don't you think? – Mister Mystère Sep 9 '17 at 13:36
• I have no idea what you tested so I can't say what efficiency they have but with a unidirectional current source there are plenty of efficient designed to choose from and realistically, one should be found that has less than 6 watts self dissipation if I contemplate some ampage and volt drop less than 2 volts. – Andy aka Sep 9 '17 at 14:04
• Dissipation is not an issue as space and cost are not at premium - this is a necessary component for the demonstration of another system. Also, I'm looking specifically for a voltage controllable bidirectional current source - what do you think of the very simple circuit I have now updated my question with? – Mister Mystère Sep 10 '17 at 20:54